"Hey R* I still need to know, will we be able to swim in RDR?" - captaincabbage
As an outlaw character who's spent most of his life on horseback in the land-locked territories of the West - John Marston is not much of a swimmer. If he has to cross a body of water, transport will be provided. John has no problem wading, but if he gets too deep, he will drown.
http://www.rockstargames.com/reddeadredemption/news/article/5081/asked_&_answered_special_red_dead_multiplayer_edition