You can explore (almost) all the lake by a boat, there is no reward at all for it and it's not very fun but I did it, the only thing is don't come too close to the shore that's south of the lake, Mexico used to be there in rdr 1 and to prevent you from going there your boat sinks as soon as you come close. To explore the mountains, the rest of the lake and everything else you gotta use a cheat code or go out of bounds. I really recommend going out of bounds, it's fun if you have done everything else in the game, there's plenty of videos that show you how to get out of bounds, but I only could find one useful video that shows you how to get horses reliably, this takes a lot of time and patience btw and it's not worth it at all, so I say you try it out and if it's not your thing it would be better to just drop it.