https://www.gamespot.com/articles/red-dead-redemption-2s-arthur-can-swim-unlike-john/1100-6461921/

Red Dead Redemption 2 is seeking to improve upon the previous game in just about every way, and thankfully, that means one frustration has been fixed up properly. John Marston was a lot of things, but he was no swimmer. The new protagonist, Arthur, is a bit more well-rounded and can actually swim if he needs to.

"Yes, Arthur can swim!" a Rockstar spokesperson told GameSpot. "It's not his preferred method of transport, but he'll do it if he has to."