I do want to offer a different position on this - many boat owners who rarely use their boats do consider opportunities to have someone stay aboard to maintain the boat. Some even allow this for free. I can't tell you how many times through the years I've encountered this -
Of course marinas may not have space for another person living aboard, but you get the boat and the slip without having to buy anything at all. Until recently, a fellow nearby had lived on a boat for 4 or 5 years free, maintaining the boat, and had never seen the owner except for the very first day he moved aboard.
It's amazing what you can find if you post an ad on the local boards, the local cork board, etc. Opportunities really are out there.
Living Aboard | The Essentials of Living Aboard a Boat by Mark Nicholas | Living Aboard.net