He's a vampire, they can't drown but my theory is that he'll fall unconscious once the water has filled his lungs. Then again this show never explained if vampire needs oxygen, technically they shouldn't need to so maybe he'll be awake but he can't do anything and he'll slowly desiccate because of lack of blood. Either way, it's one hell of an annoying fate for Stefan that's for sure.
When Elijah was daggered in the Salvatore Estate and Elena woke him up during the time the house was in her name he couldn't breathe which mean the reflex to breathe stays through but I suppose oxygen itself isn't needed. If Elijah had stayed in the house I wonder if he would have sufficated and then fell unconscious or just cointinued to feel the pain of not be able to breathe but he doesn't need to. It sounds horrible if you think about it.