You could invite some water loving dogs over, and he might join them if he sees them swimming.
Or you could do what I did, when my dog was terrified of the pool: put him on a harness and a long lead, get in, and have somebody push him in toward you. Do not do what I did, ie, not get out of his way and get clawed all over
After he swims around a little, guide him to the stairs and let him get out. Don't force him to stay in long. Let him get out and think about it.
Do this for a couple of days, then add his favorite toy. Have people he loves be in the pool, throwing his favorite toy around in the water. Then, cheer him on like crazy, and ask him to go get the ball.
It can be done without coddling and with lots of enthusiasm.