Is there anyway you can prevent him from getting to the rocks? Keep him on a leash, go to a different beach (where there's only sand)? Or can you make it a different game--if he mostly pushes a certain rock with his nose, then you sit on the beach and make it a game for him to push the rocks to you, and when he gets there, you throw the rock for him to go get again, or put it in a bucket or something where he can't pick it up?
Something like teaching your dog to play soccer, I'd think. He might not go for it... in which case you might have to choose not to take him to the beach at all.
How long have you had him? Is this a recent behaviour? I would wonder if he has a problem in his mouth and this is how he's trying to solve it?
I found this forum post that might help:
http://tinyurl.com/op6nb (I found this on a search, I'm loyal to pets.ca!)
As for the not swimming part, I wouldn't force it. See my thread:
http://www.pets.ca/forum/showthread.php?t=28205 it might offer some suggestions to getting him to swim. You might need to get him a lifejacket if he's muscle-bound or just doesn't get the concept (yes they make them for large dogs).
Hope that helps,
Melissa