Good idea when sharing with close friends or relatives, but how do you share the boat when the weather is good and you both want to go separate ways? And what do you do when one party wants to pull out or doesn't agree to pay certain repair/upgrade costs? Who's house do you keep the boat at? Just some issues to consider.
I know there are already some boat share schemes around with more expensive boats, but these are administered by a third party with defined rights and a berth.
Part of the fun of owning a boat is to be able to use it when you want, where you want, however you want. You lose some of these if you have to share.