Hi. Situtaion is as follows:
Advanced Course Certificate issued pre 31/12/04: Download Commercial Endorsement form from RYA website, send in Advanced Cert plus copies of First Aid, VHF & Sea Survival plus medical.
Advanced Course Certificate issued post 1/1/05, or none possessed: Book & undertake Advanced Powerboat Exam via the RYA or via a centre. You'll need to be in the possession of the same quals as listed as above.
More info on the Advanced Exam http://www.powerboat-training-uk.co....n%20(2006).pdf
The Intermediate Course cannot be commecially endorsed but came into existance at roughly the same time as the changes to the advanced course were made perhaps causing the misunderstanding.
More info about working commercially here
Feel free to raise questions