AT pro is great in the water, keep in mind you have to buy $100+ waterproof headphones for it to get the most out of it and go neck deep - And its also kinda old tech, its a great detector [I have one] but I don't think its the best value for the $$ these days. If $$ is an issue the Simplex might be a good option, I have one of those too but haven't taken it for a dip yet, haven't heard any negative reports of being prone to leakage.
I never been past neck deep either, don't really want to and sure don't want to invest in scuba gear just to do that. There are some goodies past the swim line to be sure, "Scubadetector" used to find all kinds of goodies with a breath tank any everything, don't know if he still posts here or dives, but diving gear would be one helluva investment I'm guessing.
Water finds are on a track to be minimal this year, for obvious reasons.