Last friday I floated boulder drop in my IK with some great folks, and although I'd been through boulder drop a few times, I struggled to keep upright and swam 3 times(special thanks to the rope out). The last time in Lunch hole, and when I got back in the boat, my shoulder hurt.  Not terribly bad, but it was sore.  Not sure when or how it happened, but somewhere in that swim function I strained it. Made it home, but in the morning it was hard to lift my left arm over my head.  I ate some Aleve and it was better, but still sore. Well, here I am on Monday morning sitting on my duff, and it still hurts.  I've read that shoulders are the weak link in a paddling activity, and proper technique goes a long way to keep injuries to a minimum, but what do I do now that I've wrenched it?  Just rest it?  I can't think of anything else

This is not for now while you're injured, but Mike found these Thrower's 10 exercises and they're really good for preventing future injury:

www.muhlenberg.edu/pdf/main/athletics/athletic_training/throwers10.pdf

He does the first 2 pages, the last 2 of those exercises shouldn't be done if painful according to the PT guy.

Good luck, I think keeping inflammation down with ice/ibuprofen like up4air said so you don't aggravate the injury by just moving your arm around is important, that's what I was told when I had a rotator cuff injury a long time ago.  I ended up getting a cortisone shot after a few weeks because it wasn't getting better and that's what helped, but hopefully yours is less serious.

Bummer man. Stay in the box.