Sonic can't swim. His creator designed him that way since he originally thought hedgehogs couldn't swim. In Mario and Sonic at the Olympic Games, Sonic even has to wear a life jacket.
So...if it was a matter of battling it out in a pool, the Sonic and Mario team would fail miserably. Sonic would kill himself.
I just found out right now. It's cute that he's the only one that wears a life jacket

Hedgehogs can swim. Sonic can't. But Sonic's a hedgehog. That's his name. Sonic the hedgehog. So why can't he swim!?!?!
-explodes (not sexually of course)-