Captain’s licenses are required to legally carry passengers for hire, having a license enables you to book charters for things like fishing, and sightseeing.
You’ll need to decide which type of captain’s license you want. There are two types:
The Operator license is for uninspected vessels (boats that hold up to 6 passengers, weigh up to 100 gross tons, and travel up to 100 miles offshore). These are also called a 6-pack license (because Operator boats hold 6 people).
The Master license is for inspected vessels (larger boats that can carry 7 or more passengers) that travel up to 200 miles offshore or on inland waters.
If you wish to carry more than 7 passengers on your own vessel, with you operating as the captain you will need a Masters License. However in this scenario you will be operating as a small commercial passenger vessel and therefore will need to have your vessel inspected by the CoastGuard.
You can find info on courses and certifications here