This business has a harder entry than most of the trade, and the hard parts come first. You cannot take paying passengers without a credential, the boat cannot carry more than its paperwork says, and the insurance that makes both usable is priced off decisions you make before you buy anything. This page is about that order, for somebody who can fish and is working out what stands between them and doing it commercially. It is orientation rather than advice: the Coast Guard and your state are the authorities, not this page.
The captain's credential comes before the boat
Carrying paying passengers requires a licensed operator, and the credential takes time: sea service to document, a physical, a drug test, an exam. People routinely buy a boat first and then discover the license is months away and the season is gone. Start the paperwork before you spend money on a hull, because the credential is the long pole and everything else can be arranged around it once it is in hand.
The vessel's paperwork sets your capacity
How many passengers you may carry is a function of the vessel and its inspection status rather than of how many fit, and for inspected small passenger vessels that number and the crew required come from the certificate of inspection. Whether a boat you are considering can be certificated for the number you want to carry is a question to answer before purchase, not after. It is also the number to put into a booking system as a hard limit.
Insurance prices the plan you have not written yet
Cover for carrying paying passengers is priced on where you operate, how far offshore, how many passengers, your experience and your claims history, and quotes vary enormously on those inputs. Get indicative quotes before committing to a vessel or a route, because a plan that only works at a premium nobody will offer you is not a plan. It is also the point at which a broker will tell you which routes are expensive and why.
Then the boring commercial part
Register the business, get an employer identification number if you will have crew, and decide how you sell: by the head on scheduled trips, whole-boat charters, or both. That last choice shapes everything about pricing and about which booking software fits, and it is worth modeling before the first season rather than after. The worksheets on this site cost a trip and price a whole-boat charter from your own figures.
Other questions people ask
What licence do I need to run fishing charters?
Carrying paying passengers requires a licensed operator credential from the Coast Guard, which involves documented sea service, a physical, a drug test and an exam. It takes months, so start it before buying a boat rather than after.
How many passengers can I take?
Whatever the vessel's status allows, not what fits. For inspected small passenger vessels the passenger number and the crew required come from the certificate of inspection, so check what a candidate boat can be certificated for before you buy it.
Should I sell by the head or charter the whole boat?
Most captains do both: whole-boat is simpler and better paid, by-the-head fills a shoulder weekday. Model both against your own cost per trip before the season, because the answer changes which booking software fits and how you price.