i do all the time. i only go about up to my knees though. its hard to retrieve your target and keep the control housing dry, but with diligence it can be done. always keep you control box dry, i cannot stress that enough. it is definately priority 1 over the target. if you dont have the cover to protect from splashing, i recommend plastic baggies. also, even if you discrimiate out the iron, it can still sound like a decent target for a few swings, but will normally recognize after a few passes. although i have found that when the target is good the signals are normally better in the water than in the dirt or sand. take your time and watch you footing because it can be very easy to lose your balance.
also if you are in a creek or river enviornment, i would stay in the shallows to avoid the current and also for better recovery. my paragrah above are my findings from a fresh water beach where the ground is relativly soft.