We've had several different partners. One couple left the partnership because they liked sailing so much they bought their own boat. Another left because his new girlfriend hated sailing. We've had partners for several years now and I have to say, it's worked out great so far. (Knock on wood.) As far as insurance, it's written in the agreement that if the partner does something that causes the insurance to go up, they are responsible for the increase for the next five years.
We have a contract that I think protects all parties pretty well, but no contract can be iron clad, and even if it is, there's always the hassle of having to enforce it if necessary. So my experience is this:
First, I trust my instinct. There was one guy, for instance, who we almost took on as a partner but something just told me it wasn't right. So I declined him. Others I've barely done a background search on and it's worked out great.
Second, I try to approach it as a real partnership, and I really try to make it work, including being somewhat flexible in the terms of the partnership if it's not a major thing. I think that way there is a better attitude all around, rather than the attitude of "I want to figure out how I can squeeze as much as I possibly can out of this other person." Instead I think all around the attitude is, "we're trying to work together; the other person is a pretty decent guy and I'm going to reciprocate."
It works for me... so far.