Have you considered Caye Chapel? They have a deep water harbor, and would be able to provide you with GPS waypoints, but you’d then need to use your tender or the water taxi to visit Ambergris or Caye Caulker.

There are also some deeper water moorings in the south, so I would advise buying the right Chart for your GPS and looking carefully for an 8'+ channel to areas that interest you?

The good news is that once you arrive, there will no doubt be many options for picking you up in smaller boats to dive, fish, snorkel of sightsee, in return for a ride on your fine vessel!

It's rarely rocket science, it's usually just math: then again if you can't do the math.......