A share is always a little more difficult to sell, and the price for a 50% share will be a little lower than 50%. Thats what I found when thinking of selling a 1/3 share. In the end, we all decided to sell; and it went very easily.
Bear in mind the psychological effect of selling a piece of something you own - the boat will no longer be yours and that may take some getting used to. make sure you have a written agreement - on cost-sharing; timesharing arrangements; who deals with insurance, maintenance etc; have an exit clause because even good things come to an end.
But look on the bright side; bills halved; troubles and work shared; the boat used not neglected due to time constraints. I've been sharing for 4 years now; its provided me with the use of a much bigger and better boat than I could afford on my own - and I'm still friends with my partner - though some would argue that the partnership should be kept on a purely business level!