you can obtain a oupv (6-pack) up too 100 ton, then a master, the master gives you the ability to operate a coast gaurd inspected vessel, which is a vessel that you can carry multi passengers, more than 6. But the tonnage is up to your personal experience. Once you submit your seatime the coast gaurd will evaluate it and tell you how much tonnage you qualify for. Once you reach, i think 500 tons you can upgrade one more time, and that is a master license unlimited tonnage
the tonnage doesn't matter in that aspect, whoever told you that was about 100 yards off the mark. To run an overnighter you have to have two captains aboard. Because under coast gaurd regulations you can legally only operate a vessel for 12 hours at a time.
as far as tonnage there is 6 pack, 100ton, 200ton, 500ton, 1600ton, and unlimited. to get a larger license 200ton and up you have to have time on a vessel of higher tonnage. as far as overnight trips you have to have atleast one master and one mate, or 2 masters because you cannot leagly run the boat for more then 12 hours in a 24 hours period