The most common (Local licenses excluded) is an Advanced Certificate of competence (gained through an exam) with Commercial endorsement.
This is where you book an examiner (which can be done through the RYA), who will assess your competence in a powerboat. YOu will also need, to commercially endorse it, an ML5 medical certificate, a sea survival certificate, a VHF certificate and first aid.
The minimum requirements for the exam are:
2 years relevant experience (As a guide 20 days, 2 days as skipper, 800 miles, 12 night hours) If you have the RYA Advanced Course Completion the seatime is reduced to 20 days, 2 as skipper, 400 miles, 12 night hours.
They reccommend you also hold
RYA Level 2
RYA Day Skipper Shorebased
RYA Advanced Powerboat Course completion
All the above is taken from G20, the powerboat logbook, and is probably the best method of gaining your license to take paying passsengers.
Not a clue on the boat requirements!