that the USCG has gotten far more careful about captain's licenses of late.... 9-11 had a lot to do with that.
The other thing to be aware of is that you need experience in "tonnage" to get the ticket for "tonnage." You need sea time that you can document (your own vessel counts) to get the license, BUT, they will not issue you a ticket with authorization for grossly more tonnage than you have ever run before.
So you can't, for example, have a 20 foot runabout, log your time on it, and then expect to get a 100 ton ticket. It won't happen.
If you don't have your own boat of significant size, this is a problem, and the most reliable way to solve it is to work on a boat in a capacity that doesn't require you to actually hold a ticket.
The OUPV ("six pack") license, which gives you the right to run with six paying passengers on an uninspected vessel, is the "starting point" for commercial licenses. The 100 ton "near coastal" Masters, which is the next most common ticket that people want from there, requires double the sea time.
The exams are not trivial to pass, and you do need to pass a fingerprint, background check, and drug screen as well as a physical (the biggie there that will get you is color blindness; if you can't distinguish red and green reliably at night, you flunk!)
I would recommend looking into the Chapman's program if you're seriously interested, but you need to consider the sea time requirement and find a way to fill it first.
BTW, in USCG parlance, a "day" of sea time is 4 or more contiguous hours away from the dock. So 2 hours of fishing in your boat does not count, but 6 hours does. Also, there are "recency" requirements (some of the time has to be relatively recent) although the total time is a lifetime requirement - provided you can document it somehow. Acceptance of claimed time is largely at the USCG's discretion, and for the 100 Ton Masters they are MUCH more rigorous about it than for the OUPV.
The easiest path is probably to GET the OUPV, work it for a few years, then go back for the 100 Ton Masters with the additional sea time you logged under the OUPV ticket - and keep a log.