Crewing on commercial yachts up to 24m in length
In most cases no formal qualifications are required. However, many skippers will prefer to take crew who have undergone training to at least Day Skipper level. If you look for work through a crew placement agency, they will often recommend that you hold a Day Skipper certificate, preferably supported by training such as the Diesel Engine and Radar courses. Details on all these courses can be found in the links above.

Crewing on superyachts
You may need an MCA Yacht Rating Certificate as a certain number of crew on board are required to be qualified. For this you should hold the RYAs Day Skipper or Watch Leader and Competent Crew certificates and will need additional STCW training in sea survival, fire fighting and prevention, first-aid, personal safety and social responsibilities. See the links above to learn more about STCW.

Full details of Yacht Rating training can be found in the Guidance & Regulations section of the MCAs website accessed via the link above.