Lab won't swim
Having another dog along that enjoys swimming would also be of help, as the competition will increase the drive to get the toy or whatever you throw.
i've heard that big water can be intimidating for some pups so if you are not already i'd take him to a smaller pond, river with slow moving current, or a narrow channel in a lake. you can use cheetos to encourage him, just throw them out a little further each time and wade in with your dog. also, having a dog along that is an experienced swimmer and excited about the water may also give pup more confidence.
do not rush it and force him into deeper water, the dog has to make the decision for himself.