I would keep it simple:
- if you're not friends, or willing to become friends with the couple, don't bother.
- officially buy the boat from them, and assume responsibility for it on paper - your name on
ownership,
insurance, marina, etc
-
draft a letter of understanding that they will get X days of use per season, and in return they will contribute Y% to the marina
fees,
insurance, annual
maintenance. Since you're the owner of the boat now, upgrades are your responsibility and cost. It's not fair to stick the old couple for the cost of a new sail or
engine replacement if they have only a few years left to use the boat. Such considerations would have been addressed by the
purchase price.
This keeps it clean, they have their
money, you have your boat, you both share use. Any other arrangement, like
legal co-ownership just seems more hassle than beneficial. Someone always ends up feeling screwed.