If its a Maravia and the boat's not already encapsulated, then the boat's probably pre-1988 and may not be worth fixing. You may need to just get used to topping it off in the mornings. One thing to be wary of is that if its an old boat, the seam tape on the inside of the tubes may be coming off. This will be a difficult thing to tackle and you may need to "rebuild" the boat which may take effort than its worth.
I've never used Gacoflex myself but have heard that the preparation is where you really have to do some work to make it seal well. A better place to cast about for info would be on Boatertalk's Raft Zone (http://boatertalk.com/forum/RaftZone
). I remember seeing a post in the last couple of days about painting boats. If you visit the site, you may be able to do a search on "gacoflex." You could also give the folks at Downriver Equip. a call - I think they sell the stuff (303-467-9489).
An alternative to painting the boat would be to take it down to John at Inflatable Technologies (303-619-332
and have him encapsulate the whole thing for you. It'll be more expensive than the paint, but the boat will be a lot better for a lot longer. John should be able to tell you if its worth encapsulating to begin with, and will be able to tell you about your options with prep and painting also if you decide to go that route. John probably knows rubber boats as well as anyone on the Front Range and will be honest with you even if it means that you wind up taking your business elsewhere or having him do less than you originally intended.