Ok, i have never used on in water but it is my understanding that they are waterproof.
I would go as far out as you can wade in known swim areas then. I say that because most beach hunters (that I have seen) play in the sand. I have only beach hunted from time to time.
Sorry, I wish I could say more, but I don't know much about the AT Pro.
There are a lot of beach hunters here and I am sure they will weigh in. But you have a great all around machine there.
I think the machine should do well, Can't get the unit underwater. I generally hunt the high sand, looking for old stuff and have done pretty well. A friend goes in the water but we're in the mid Atlantic and I won't be in the water until later this year. Some people look for "holes" in the water stuff congregates in.
You can take the At pro up to 10 feet under water. I'm not going to dive with it though. I'm going to try the dry sand and might get out to waist deep. Which do you think would be better? I'm going to get out early before the family gets up. I'm planning on getting up at sunrise.