charter fishing boats

I think the biggest qualification is that you work CHEAP.:-o

You would have to know boat engines, how to care for tackle, bait hooks, run lines, etc. for charter fishing and many snorkel tours require CPR/First aid certification, swimming/snorkeling ability, some boating knowledge and experience. Ditto for daysails/term charters, depending on size of yacht and type of charter, some sailing experience is obviously preferred.

Enthusiasm and willingness to work hard for cheap plus tips goes pretty far, too. All that Alana mentioned is bonus. I know several folks who work on sailing/fishing/snorkeling excursions part-time and in a restaurant part-time. One thing they love to do, the other one makes them money.

You must be able to pass a drug test to be crew on any vessel operated by a USCG licensed Captain taking paying passengers - and be a member of a drug Consortium for random testing -- if you are working on a boat that does International routes [BVI] you must possess an STCW 95 certification - there is some turnover, it is long days and very dependent on tips, seasonal and the entry level jobs are hard work.
If you persist as a dedicated crew and improve your skills and follow a path towards professional yachting it is a rewarding industry.