I feel like if aquatics were added, it would make sense to split the arenas.
However, if that ends up as not being the case, i feel the aquatics would have possible exclusive moves where they could possibly lower crit chances, a move similar to the pokemon move “pain split” to where they can even out their health, or even use something that out-prioritizes swaps and turns into a rampage doing so. These moves would be called:
Demotivating Strike/Impact/Rampage: Deals 1x,1.5x,2x damage, lowers the opponents crit chance by 25% for two turns
Level off: has priority, evens out hp to meet at the same percentage remaining
Flee Prevention: deals 1x damage, bypassing shields and armor. Damage doubles if the opponent swaps and acts before the swap