Try to slowly condition him to the water (giving treats and slowly getting closer and closer to the water while giving treats), but there are some dogs that just don't like swimming, mine for example hates the water unless it's quite hot outside, then she jumps right in
I have a dog that just learned that he likes the water. He appears to have some lab in him, but I didn't know for sure. Lucky for me, I live near a lake that has a very shallow shore line. I started off by just letting him check it out from the edge. Then I rolled up my pants and walked in a bit and let him follow. After that he has gradually become more and more used to the water and would even go out some distance on his own, but he wouldn't go in deep enough to require him to swim. Last weekend, he swam for the first time. I just walked out into the water and encouraged him to follow.The water was just above my knee so he had to swim to keep up with me, but he could easily turn around and walk on the bottom again. He did really well so I got a toy and threw it a short distance. He really seemed to love it. I have read/heard stories of people losing their dogs because they took them boating and assumed they could swim, and some of the bully breeds are so solid that they are pretty much sinkers so be careful. If your dog seems hesitant, I wouldn't push him/her too much, as with all things, you want it to be a positive experience so that even if your dog doesn't "like" the water, he/she won't become afraid of it.