I use an AT Pro. I love the waterproof feature. I can hose it off after a hunt, wade the creek with it & even lay it down in the water while I dig. Just make sure to keep the o-rings greased (battery cover, headphone & coil plugs). Use a good silicone grease to keep it waterproof. The dealer told me to use vaseline, but I've seen petrolium-based lubricants destroy o-rings, so I won't chance it.
The stock headphones are fine in a light rain, they are water resistant. No reason to buy the water proof headphones unless you plan on wading or hunting during downpours. The speaker in the head is also waterproof to 10 feet, so worst case, you can wade (or detect in heavier rains) without any headphones. Just make sure to close off the headphone jack on the detector head with the provided cap. It's attached to the headphone cord, and has a foam washer inside that you should keep lightly greased, as well.
The AT Pro does very well for depth, but it's very noisy in trashy ground. I typically hunt in Pro/Zero with nothing notched and iron disc at 20-30, iron audio off. When I get a decent hit, I turn the iron audio back on and re-sweep the target to check for iron falsing.
Last but not least, when you're hunting wet ground, recheck your ground balance often, especially if you are in heavily mineralized or trashy soil containing a lot of scrap iron or iron minerals. Any iron oxidization in the ground can really come alive in wet soil, especially in compact soils. We've got a lot of magnetite and maghemite in dense clay around my area, and when the soil is saturated, you might as well be detecting over a buried sheet of tin roof - no depth and constant coil overload.
HH
Love my machine rain or shine runs like a champ. also, plan on using in local creeks and at the beaches.