The advice given by Marmot is excellent I would agree that far better to obtain the internationally recognised commercial STCW tickets (certificates) than the national restricted tickets. To get these tickets you have to proper seatime on commercial ships or large yachts This also applies to engineers.
You also have the possibility of a proper career path if you wish.
The RYA tickets were originally designed for the private yacht owner run a small yacht safely. The Commercial Endorsement is just a stamp used to fill a gap allowing holders of the RYA tickets to work commercially on small charter yachts.
This has lead to situations like this:-
http://www.maib.gov.uk/publications/...dy_candida.cfm