First off, communication is important, you need to tell him you are worried for his safety, given the scares in the past. However, I didn't hear about a single hospital trip or coast guard call, so, from my point of view, the scares you discussed weren't life threatening. He's a grown man, and he has stayed alive this long, this boat won't be the thing that kills him.
The expense isn't the issue, so,let him buy the boat, you are his DW, not his DM. If the expenses to take care of a fishing boat are the only hobby he spends money on, you have gotten off cheap. I spend more than that on lunch.
Are you sure that you aren't jealous? Of where he wants to spend his time? Or jealous of the fact he has a hobby and you don't? Whether it's safety or some other reason be sure you know why this bothers you and discuss it with the DH, and maybe you can reach a compromise you both are happy with.