The AT Pro will only work on the dry sand. If you want to go to the wetsand you are going to need a PI or multifrequency detector. Since you already have VLF, I assume thats what you are looking for. You only have a few choices - CZ21, Excalibur, or CTX. There are some others out there that are not waterproof but I really dont recommend those for salt water hunting when there are waterproof options. Look to pay $800-$1000 on a used CZ21/Excal as its prime season right now. New they are $1500. A new CTX is $2500. Beach machines are usually the most expensive because they need to be resistant against salt corrosion as well as multifrequency.
As far as PI detectors go, I would only recommend them for certain beaches and with certain conditions. So that depends when/where you are hunting. If you wanted to go PI there is whites DF, garrett sea hunter, and tesaro sand shark.