Many years ago I was an EMT, and I definitely saw my share of things, and you're right, it's entirely different when its a loved one. Well at first anyway, but after that initial reaction, knowing what to do can go along way to helping the situation. I'll admit that having confidence in your actions based on experience really helps in those situations, but You can't go wrong by learning more about what to do.
One of the things I'd suggest concentrating on learning is when is someone in an immediate life threatening I situation vs. when they're in a lot of pain but not in immediate danger, which is way more common in my experience. It'll be easier for you to remain calm if you KNOW this will turn out ok.
Stabilizing a compound fracture of the tibia is not that hard and you can surely learn how to do it and what to do to minimize blood loss.
What would be the best place to go learn? I don't know. Somewhere there must be a course specific to boaters. Of course the difference is the amount of time you could be the only medical help.
Know anyone in the medical profession? Ask them to help you out. I actually got my old partner (now nurse) to come out for the day and give my wife a crash course on what she can do if I get hurt. She's been looking forward to stapling me ever since