Question about Pantherophis obsoletus

[LadyWraith]

This weekend while camping, we watched a big black snake swim from the opposite shore of the lake (about 1/4 mile). It came onto land right next to the dock where we were fishing. We didn't know at the time what species it was and luckily it went under a rock pretty cause other campers were convinced it was venomous and needed killing :/ We got a good look later when it was warming up on the pavement right by our truck. My ?... are these generally found swimming? It took me awhile to figure out what it was cause I've never seen a rat snake going for a swim lol.