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The rocks are placed in the corner and smash Pokemon upon send-in, so washing them won't really do anything to their purpose. It's also unlikely that the waves will have sufficient power to wash them from that distance. Spikes, however, deal damage based on physical contact and so washing them to the side would negate damage.
A note to those waiting patiently for me to ref: Thank you for your patience, it means a lot. I'm planning on heading home for this weekend, and I'll try my best to ref as quickly as possible during that weekend.