Well, kinda. First, gross tonnage is a purely calculated value based on the internal volume of the vessel, minus a whole bunch of exceptions (and exemptions). Licenses for vessels carrying 7 or more passengers are based on tonnage and require 2 years (720 days) of seatime (4 hours or more spent away from the dock, with a few exeptions), some of which you may acquire on your own vessel, and some of which must be acquired on a vessel of a similiar tonnage for the license you are seeking. Vessels for hire carrying 6 passengers or fewer, regardles of length or tonnage require a 6-pack (don't use that term at the Coast Guard office), or Operator of Uninspected Passenger Vessels (OUPV, use that term) license. Sea time for a 6-pack is 360 days on any vessel.
The full requirements may be found on many USCG websites. The website for the Marine Safety Office/Regional Examination Center (MSO/REC) for Honolulu is the simplest to navigate, and the most friendly for folks who don't regularly navigate the web of confusion of the Coast Guard websites.
That will cover the basics of getting a license, but that is the easy part. The hard part is having the confidence and skill set to safely navigate that thing through whatever Mother Ocean sends your way, and understanding and accepting that you have complete responsibility for those lives on your vessel. There are way too many "Paper Tigers" out there who may not have a complete understanding of how their vessel operates, how it handles in a cross-wind, how hard to back down to get the stern to kick over, what to do in the case of a fire, unruly passenger, unsafe diver, The list goes on and on.
The Coast Guard requires the apprenticeship time for you to learn how to deal with these things. How many bilge pumps have you had to take apart because it sucked up a rag and the vessel is taking on water? When water seeps through the wheelhouse window gaskets in heavy weather and gets the GPS wet, will you still have the skills, backup instrumentation, and instinct to find your way home? When you get a bad load of fuel, and you are plugging the Racors left, right, and center (assuming you have 3 engines), will you have enough spares to continue on home, and will you have the fortitude to keep going, even though you are destroying $5,000 worth of injectors. Oh, by the way, none of this ever happens on a sunny day in Casco Bay, it happens when the wind is blowing 35 steady out of the Northeast, with a 10 foot wave on a 20 foot swell that NOAA didn't tell you was coming. Oh, yeah, and all those lives are YOUR responsibility. Is your sea time on a 17 foot Boston Whaler that goes on the trailer and to the engine shop at the end of the weekend, or do you spend your evenings after work at your real job cleaning bilges and changing spark plugs?
Do you have a commanding voice and presence? Will folks in a full on panic listen to you and follow instructions when the ca-ca hits the rotating air mover? How are you going to handle it when all divers but one surface, and the one is the newly certified husband of one of the women on the boat, and she is screaming at yo to find her husband so loud that you can't think straight, and your divemaster discovers his lifeless body on the reef in 60 feet of water? Not all days are like this, but some are.
With all that said, I wouldn't trade the job for love nor money. There are very few greater rewards than when a newly certified diver breaks the surface and goes on and on about the fish they saw, or when Mother Ocean hands you a whale shark, or manta ray, or when you look at the checking account at the end of the year and see that you actually made money. Some of the greatest people in the world are divers. I have seen divers who paid full price for a trip give up a fin (and their dives) so that a newer diver will continue to be able to dive.
In closing, there is way more to being a Captain than meeting the time, physical, and drug testing requirements of the Coast Guard. Run for a summer on someone else's boat before you decide to make the commitment. PM me if you have any specific questions.
Frank