River Otter I believe.
We had a couple show up on our small lakes last spring too. Fun to watch, no doubt about it. They play in the water like 10 year old kids. They disappeared come June though, not really sure why; boat traffic maybe?? Give them some room though, they are still wild animals that have been known to be a bit territorial with swimmers in the water; nipping at people at times. We never saw it but read reports of such. It is amazing all the different critters we share the water with.