I feel that a Pokemon stadium should act for universal battling, much like in the games. I say this a lot, but even without hovering, water Pokemon could seem perfectly capable of moving around on land. If they're fish or similar aquatic creatures, they could hop on their bellies (in the first mini-movie, Kasumi's Tosakinto managed to do just that), or tails (Kingdra line), shells (Shellder, Parushen, Pearlu), etc. If they're extremely large, girthy Pokemon like Laplace or Whaleou, they drag themselves using their fins like a sea turtle. Some could also slither like a snake (Huntail, Sakurabyss, Milocaross... Gyarados also showed to do so in the anime, but I don't see the point when it's a Flying type that's based off an oriental dragon). Sure, their movement might seem greatly restricted compared to being in water, but at least they could move in any case (and really, they're no less worthy of participating in a land battle than a Tsubotsubo or a Foretos). If nothing else, they could stay in one place and fire projectile attacks.