Hi, aj,
Your dog should be trained to come back to you when you call him! They can get "overly tired".
Here is a link about training dogs to swim (I know yours already can), but scroll down about halfway to "tips for swimming".
It even suggests having a "doggie life jacket", if out in a boat far from shore.
http://www.triplecrowndogs.com/train...Tip=16&Topic=2
I would be concerned if he doesn't come back to when you call him. Best wishes.