There are many on here better equipped to answer than I am but here goes:-
Firstly - you dont say if its a PB2 or an Advanced Powerboat. If its basic then you are only allowed 3 miles from port!
To get it endorsed you need the following:-
a) ML5 Medical
b) Sea Survival Certificate
c) First Aid Cert
d) VHF Cert
Because of the 3 mile rule - pretty much the only place this is useful is the Solent.
You can see most of this using this link here.
I get thoroughly confused about the rules governing the commercial endorsement to "Advanced" but presently you cannot get an Advanced Course Completion Certificate endorsed. You need to take another exam before you can have the Advanced Commercial Endorsement. If you had the Course Completion Certificate before Jan 2005 then it can be Commercially Endorsed. Easy eh?
However, with Advanced you are allowed 20 miles, so its more usable.
Have a look at the application form here.
There is work for PB2 Commercials - I could have worked all of last summer but chose to restrict it to a couple of days every week as I am a generation older than you, and I didnt fancy 7 days on the trot at 30 knots in a Rib around the Solent.
I do think its a tough way to make a living from a financial viewpoint. That doesnt worry me now, but at your age I couldnt have considered it as I had kids at School/College etc.
Good luck anyway whatever you do!