Personally I would probably keep the AT pro for the beach and water hunts as it's waterproof. This of course would depend on your personal preferences and the results you were getting. If an upgrade is in order, definitely go with the multiple frequency machine if it's in the budget-but they can be expensive. And if you're looking for waterproof too, then likely double the price. For example, an E-trak goes for around $1100. But the waterproof all terrain Minelab CTX 3030 or Garrett ATX will set you back about $2500.
For dry land performance I'm partial to my Minelab X-Terra 705 which came with 2 coils. You can get the single coil unit for around $729. This is single frequency at a time, but can choose from 3 available frequencies by changing the coil, which there are several to choose from. No detector is ever going to be a "perfect" machine, but I've had really great performance from mine and have picked up targets over 2 feet down. So far my deepest quarter came from about 14 inches, and I often find tiny 1/8 inch diameter fishing sinkers at a foot down. The machine is extremely sensitive. Minelab also makes the X-terra 305 and 505 which of course are much lower cost (about $200 increments) with a few less bells and whistles but still very good machines.
Hope this helps!