[capta]

You will also need a real DOT, in depth physical exam, eye exam, hearing exam, drug screening and to join a random drug testing consortium.
If you have any physical problems at all, you will now need specific testing to insure you can do the job (stress testing, EKG for cardiac problems, for instance), however minor or congenital. This can get very expensive.
You may also be required to get 1995 STCW, another few hundred bucks.
As for gaining knowledge or refreshing your knowledge, be aware that the purpose of the test preparation is exactly that and no more; preparing you to pass the test. Since the tests are mostly multiple choice, you will be taught to recognize the correct answer, not about the latest changes to the Colregs or nav aids that are not on the exam.

 

[capta]

I have thought it would be a fun "early" retirement job (though I seem to not be getting there on the 401K for the early part) to be able to take dive boats out and and other tour type boats. Would a 100 ton be enough for that? I know I when I went on a snorkel boat tour in Maui, my brothers friend was a part time captain for them, and he seemed to enjoy it. I don't think he really needed to work, he spent half the year in Montana and half in Maui. It was a long term relationship with the tour company and I understand it is not exactly like they are desperate for folks to do the work. But I could start working on the relationship soon enough.

It really depends on how much responsibility you want to take on. On dive and snorkel boats (you may be required to hold a divemaster's certificate) you not only have the responsibility to keep your passengers alive and safe while they are on the boat, but in the water as well. You have to quickly assess those who may not be safe in the water and insure that they always have a crew member's attention, while making sure the others are safe as well. Doing dive charters in St.T I found as a general rule, that out of 10 divers, 1 was there to commit suicide, 2 should never have been certified and one other hadn't been diving in so long they had forgotten the basic safety rules.
Motor tour boats are the easiest job for the money and big schooners on several hour day sails are IMO, the most fun. But no matter what you operate, if you have a good crew, you have it made.