I would have said the Minelab Excalibur,of course the main reason its totally waterproof,based on the very successful Sovereign model that most folks often consider possibly the best all round beach detector going,of course the down side with that model is the fact that you mentioned it has to be waterproof.This last request alas reduces your choice down a little and also possibly boosts the price up a lot.
Also the best option is a multi-freq machine as these work best on the wet sand,some folks say that a single freq machine do work,thats not for me to say that they dont but its often seen the multi-freq is superior,the other tried and trusted machines are based around the Pulse platform but they also have good and bad points as well.
One thing you have not mentioned is your budget,no point mentioning dedicated expensive machines if your budget is on the low side.
