i think that the fact that an Ally and Enemy system has been integrated (variables, not AI) it would make the possibility of team death-match much easier. so this could be a feature, or at least one i would like to see
ragdoll would be a bit iffy online, as both ends will probably not compute the fall 'exactly' the same, thus the body could be in a different place and then the collision areas would be very different. If it were possible to disable the collisions for MP, i think it could be a cool possibility.
i really don't think there's anything wrong with the water that would cause a problem in MP, besides the physics of objects and buoyancy (but then again, there wouldn't be physics for those entities in the first place!)
then again im not the best programmer so my 'could's mean nothing, best to just wait and see