Like everyone says, do the first aid course, you never know when you might need it.
If you have done a course, when was the last time you updated? DirtySanchez is right, there have been some changes to CPR recently and some places are already teaching the new method. QAS wasn't when I did my refresher two weeks ago, but said it was going to come really soon. Doesn't really matter, as long as you know how to do CPR.
First Aid and a working radio are two very important things to always have on board. They can mean that you keep someone alive and get the right kind of rescue arriving quickly.
A few years ago I was on board a boat where a mate of mine had a nasty diving accident off Cape Moreton. The only reason he is still with his wife and young son is that someone kept him going with CPR and the medevac chopper got there PDQ. We thought that he was dead. It was a pretty emotional time to see him back on his feet again.