You might ask the vet about giardia prevention, if there is such a thing. I know it's fairly common in the midwest, and it's an infection caused by drinking water contaminated with some kinds of germs that wild critters (like beavers) carry. It can cause lots of digestive problems and hang on a long time. Probably just not letting your pup drink from streams or lakes is the best prevention, but...?? I'd probably want to hose him off with clean water after he's been in the lake too, but that's just my compulsion for clean dogs. *smirk*

He'll have a ball swimming, I'll bet! Get lots of pictures!