When it comes to the two types of metal detector the main difference in terms of being waterproof is that one deals with the being in saltwater a lot better than the other. The pulse induction type of detectors won’t be affected by a high mineral content which can come from the salt in the water as they will be able to see a much clearer view of the ground below them.
The VLF type of detector though will be affected by the salt minerals and that clouds its signal so therefore it can’t be as effective when it is placed in saltwater. If you’re heading out to the sea then make sure that you are getting a pulse induction detector otherwise you won’t get great results as your signal will be clouded by the minerals if you used a VLF detector.