Are you planning on staying at a marina or hooking up to a mooring ball? if you're not going to stay at a marina, how do you plan to cook, store cold food, go to the bathroom or shower? yes you can do all those things on a 21 footer for a short time, kinda like camping, but long term? That's a rough lifestyle. Speaking for myself, I wouldn't go with anything less than 25-26 feet and there is a huge difference in interior space between 21 and 25 feet.