We used to have a pool and Abby wouldn't get in. We finally forced her in with us a couple of times b/c I wanted her to know where the stairs were in case she fell in when she was running around it. I think she doesn't like the immediate drop of the pool floor. She never got in again on her own
She wasn't keen on the bay either and I pulled her in with her leash to show her that she could walk in and that is wasn't a complete drop off like the pool. Now that she knows the bay is like a ramp it is hard to keep her out.
If you know of other lab(s) or dog(s) in general have them over for a little pool party. In my experience the best way to get a lab to do something is have him or here see other dogs having fun doing it.
We have an in-ground pool and our younger dog, Bodie, would not go in. He never really liked water, wouldn't even go in the kiddie pools before we got the in-ground pool. Our other dog Sam loves water. Bodie would run around the pool while Sam swam, and you could tell that he wanted to go in. We tried for about a year to get him in, but he was too scared. Then I saw an episode of Dog Whisperer, where Cesar took a dog into the pool on a leash. I put Bodie's collar and leash on, and he went right in! I think he felt better being attached to me. For the first few times, he would only swim about 1 foot out and then turn around. We took off the leash after a few times in the pool, and after a while he would swim across the pool. Bodie likes swimming now, but not as much as Sam. It definitely made him feel better to have us in the pool with him.