Old guys should surf - or do something in the water. That said I fried my shoulder after two consecutive days of boogie boarding a few weeks back. The second day I was doing a lot of one armed paddling (left hand on the front of the board, right arm paddling) and worked too hard for too long. But some chiro and a shot from my ortho settled the rotator cuff down.
You could also use some wind on the water - this was yesterday's class:
Turns out you can race these things. Uh oh...