From some experience with clients in co-ownership arrangements, everybody here is right. They can be very good, sharing the ownership costs, or they can be a nightmare. A few points-Make sure your co-ownership agreements covers as many things as you can think of, no matter how small or trivial they may seem to be. Written upfront and clearly understood eliminates a lot of issues down the road. On maintenance and upkeep-who is responsible for seeing it is done and who does it? Generally, at least with the larger boats my clients have been involved in, all maintenance and upkeep is outsourced to professionals and the costs shared. In many cases, a professional yacht management company is used. That eliminates a lot arguments about what needs to be done and when it needs to be done. To me, it is not a great idea if one owner intends on doing a lot of the maintenance and upkeep himself. That leads to issues of how he gets credit for his time for doing it? Does he get additional cruising time? Does he actually get paid? Builds up some bad feelings when one owner spends a disproportionate amount of time on the boat. Also, doing one's own maintenance may create liability issues and, if something done fails, disagreement on the quality f the work and the cost of fixing it again!
Again, it can work, but as was noted, all parties have to be prepared to put a lot into it.