At that age it is important to introduce things gradually so he doesn't acquire any permanent aversions to water or even worse, guns. Labs are not particularly sensitive usually and you should be able to get him used to both in short order. It still needs to be done step by step to be safe. If you don't have a training book (or video) to tell you how to introduce water and gunfire gradually then you need to get one. If you can get him used to guns, water and birds then it should be fine to let him tag along hunting. You can't expect very much though and I wouldn't take him on serious hunts with other people because he's going to mess up (no way around it at that age) and you don't want him to associate hunting with getting in trouble.