Peter White from Seafever Sea School runs a very good first aid course which includes getting wet and dealing with casualties in the water and on boats. Its 2 days instead of the usual 1. I have done both and feel that the extra day spent in and on the water is very worthwhile.
i did a one-evening St Johns course which was accepted for the yachtmaster.
that's a minimum and i would have been happier with a proper ship captain's course but they put so much emphasis on the exam rather than teaching that i feel they're poor value.