Why is every so upset with Levitate being a set ability? People are actually complaining that using said combination of types will make Levitate invaluable because it can deal with a lot more threats. Yeah, that would suck if CAP10 was doing it's job. Last time I checked Levitate was a fantastic ability and it only adds to the utility of either types. Helps it switch in and take down many of the EQ spammers in OU. Also protects it from Spikes and TS so it can't be hindered by those which would help it work as a stall counter if need be. At the same time Poison could use a non-Levitate secondary ability and soak up the TS which only adds to versatility. Makes sure it isn't trapped by Dugtrio/Trapinch as a final note. There's literally nothing negative about giving it Levitate.
The negative thing about Levitate is being locked into it as an ability choice. Yes, Levitate is a great ability. But there are tons of other great abilities as well, some of which are more unique to CAP10 and much more interesting. The CAP process is about exploring things that aren't in current OU. Electric/Poison is unique typing, but a tank w/ levitate is not. Electric/Water has been done before, but only once, on a pokemon with a cool niche, but not an effective, consistant OU pokemon. No one ever said Levitate was a bad ability for CAP10, merely that its bad that CAP10 depends on Levitate to function.
Anything that's weak to EQ will never effectively bring down DD Mence/Gyarados/Tyranitar without proper priority, stats, etc. Metagross has the incredibly bulky Defense stats of 80 HP and 130 Defense and without a + nature and tons of defensive investment it's OHKO'd by +1 LO Earthquake from neutral attacking natures. Still can't touch SD Gliscor who will OHKO with STAB EQ. Still can't counter CB sets on anything carrying Earthquake. Still can't switch into Specs/LO Heatran without heavy prediction. If you avoid Levitate with Water/Electric you'll just be forced to use another ability that deals with it's Ground weakness so you're not solving the problem.
First of all, you say that this thing could never counter Gyarados, Mence, et al., if EQ weak, without the right stats. You can't really counter them anyway w/o the right stats or other stuff. But Water/Electric is a better bet to counter both T-Tar and Gyarados, due to its typing. A Water/Electric pokemon will need less investment in its defenses to beat Gyarados as it resists waterfall. A Water/Electric pokemon will need less investment in its offensies to beat T-Tar because it has a STAB SE attack.
And if a DDMence gets the chance to use DD, and you don't lower its attack with Trace or Intimidate or something, your more than dead anyway. STAB Outrage has a BP of 180, while SE EQ has a BP of 200 - not much difference. Sure if your Electric/poison had Levitate it could take an EQ, but neither is gonna take an Outrage without serious investment + something extra, like Trace/Intimidate for the switch in, and/or Ice Shard. The former option does not work if you already have Levitate. So even without the Ground weak, Electric/Poison w/ Levitate will fail to counter DDMence.
I'm not 100% against Water, it's just using Ice resistance as arguing points is... odd. Ice is almost never STAB'd so it won't be powerful enough to make a difference. HP-Ice from Zapdos isn't going to do much to CAP10 regardless of resistance or weakness. And if it is STAB'd, it's Ice Punch from Weavile. Gasp. So Ice really only has a single threat and a pretty lame one at that.
So the only users of Ice moves are Zapdos and Weavile? What about Tyranaboah? Sure, its not the most used T-Tar set, but it always has and Ice Beam. What about Starmie, who carries Ice Beam 63% of the time? Or all the Ice Beaming bulky waters? Ice is a very common attacking type, and while the hits aren't normally very powerful, the difference between 20% and 10% of your HP is HUGE. An Ice resistance gives CAP10 a bunch of little hits that it can come in on and brush off easily.
Fire is more viable to an extent but with only Heatran and Infernape firing off STAB shots it's not that impressive of a resistance. Heat Wave from Zapdos struggles to 2HKO standard Gliscor who has terrible Sp.D resistances. The only other true mentionables are Overheat from Rotom and Fire Blast from Salamence. These are much better arguments but still not the best as most things with a decent Sp.D can deal with them and moves like Shadow Ball and Draco Meteor will be feared much more anyways. Random HP-Fire's and Flamethrowers on things that don't use them well (Blissey, lead Azelf) are the only others and don't really make it worthwhile.
Again, you underestimate the difference how important the amount of health you save is. When battles come down to damage rolls, that 10% of extra health will be vital.
An d Just a note, but a resisted Rotom Overheat is a pretty awesome switch in. Not only do you take little damage, but Rotom is forced out. A neutral hit is much more crippling, making you much more vunerable to revenge killing.
Water is better, but still not over the top fantastic. Helps against Gyarados but it can still abuse your weakness to Ground. Helps against Crocune, but it will still set up all over your ass unless CAP10 gets Taunt, a fantastic Attack stat, or something else to break it but that would be poll jumping. Helps against Vaporeon, but so does immunity to Toxic, any amount of special bulk, and STAB Electric attacks. The main enemy would be Empoleon, but it's uncommon. However without a Water resist it's incredibly difficult to bring down Sub/Petaya (though weakness to Grass Knot doesn't do it too much better). Only other mentionables are Latias and Swampert who both have much more threatening attacks.
Water is an AWESOME resistance, if only for Gyarados. Gyarados isn't even sure to be carrying EQ. EQ usage is going down, as it isn't the greatest typing to pair with water. I would much rather be able to switch in on a spammed Waterfall with Trace/Intimidate than predict an EQ with Levitate, that is only there half the time. A Water resistance makes this a sure-fire counter to half Gyarados (all if it gets good speed, and checks them if it doesn't). Levitate makes it a shaky switch in to one half of all Gyarados. And again you forget Starmie. Starmie is a potent threat and one of the most used water types. A water/electric CAP is a hard counter especially if its light, to avoid random Grass Knots. An Electric/Poison CAP is weak to one of Starmie's stab moves-the lesser one alright, but it still sees use. CAP10 is dead weight if its easily revenged, and water helps prevent that, as it resists common moves of revengers, like Heatran's FB, Starmie's Surf, Hydro Pump, and Ice Beam, and a very lovely x4 four resist on Scizor's BP.
The same can be said about a lot of Poison's resistances, I am well aware, but most of those are STAB attacks. Scizor uses U-Turn and Bug Bite and Celebi uses Grass. Only two real worthy mentions, but being able to counter Scizor for sure will be extremely useful.
I don't know where you got the idea that the ability to switch in on U-Turn means you can counter Scizor. Yeah, you resist it, but they can switch straight away to a CAP10 counter. Scizor will have done its job. CAP10 will have not. And don't say anything about Bug Bite, as it is inferior on CB Scizor (the best and most danergous set) to U-Turn and is only used 11.8% of the time.
Electric is more than mentionable, since Zapdos, Jolteon and Magnezone lose their main powerful STAB and are forced to use lesser means in order to damage CAP10, like the apparently threatening Heat Wave and HP-Ice/Fire which won't matter if your set is tailored to counter those with heavy Sp.D investments.
Here's a point I am willing to conceed. The worst thing about Water as a secondary type is the lack of an Electric resist.
Resisting Fighting is a much much more viable point however. CAP10 will not be able to counter Lucario without it. If a +2 LO Close Combat can OHKO defensive Skarmory then there is no way a Pokemon who isn't meant to have super high defenses is going to be able to take it. And what about Machamp? The only way to really deal with it on most teams is to carry a Ghost type or hope you can stall it down with Zapdos or Gyarados. And Machamp carries attacks for both those types of defenses. Not to mention CB Machamp would tear Water/Electric typing open with STAB Dynamic Punch as it does to everything without a resist. Breloom also crushes CAP10 without an issue, resisting both STABs and easily smashing it with either Focus Punch or Seed Bomb (while Poison resists both) and I really don't see anyway it can fix it.
First of all, don't talk about countering Machamp with a Fighting resist. Dynamicpunch will get the better of any non-ghost pokemon without those super defenses. And yes Lucario can OHKO Skarmory, who has a measily 65 base HP. I don't imagine CAP10 getting that little HP. And its not a good idea to switch any pokemon in on a +2 CC, even with a resist. If you want to kill a SD Luke, you either switch in on the SD and outspeed, or you revenge it after a CC.
Breloom is a completely different issue. Yeah an Electric/Poison CAP could switch in on his attacking moves, but what about Spore? Again we come across the issue of locking ourselves into Levitate-we essentially have to have Levitate as one of the two abilities, eliminating a spot for something like Insomnia or Trace, which could help with specific threats. This pokemon needs customability to conform to the concept. When you fill in levitate as one of the abilities, you lose some of that customability by giving CAP10 an ability that helps out most everywhere.
I see both side's statements but Poison shines clear imo. If Fire/Ice is such a valuable resistance to have there is always Thick Fat. Thick Fat AND Levitate? Do you really want TWO type specific abilities? The opponent will never know you're ability without proper scouting and it should only be brought out to deal with one specific threat. Most people will know that when you switch in on Gyarados, you are going to try and counter Gyarados. And Levitate is one of the easiest abilities to scout for-just set up some Spikes or TS. If CAP10 hits/absorbs them, no Levitate. This means the only logical resistance being missed out on is Water, which is a nice resistance to have but not 100% necessary. I'd much rather take the Fighting resist > Water resist. W/E works but it just doesn't compare to P/E imo.