It took forever, but ever since his ball drive sort of clicked a little while back, he's been doing a LOT more in exchange for his ball - including swimming! I really thought he was just never going to swim. For the longest time, he was very picky and the few times he went into water, he refused to go deep enough for it to even touch his belly. But he decided he really wanted that ball when we went to the lake yesterday and finally decided that swimming wasn't so bad when there are fun things to chase into the water.
So for anyone who wants a swimmer and is not having luck with their pup, don't lose hope! I never pushed him into deeper water than he was comfortable with and always made it fun. Looks like it's paying off now.