It may vary state by state, but I know in Mass. you don't need a title. I suspect it's that way most everywhere (if Mass. doesn't require the extra bureaucracy, it's hard to imagine someplace that does) , but I certanly don't know for sure.
In Missouri you need a title to register a Sunfish (any boat over 12' long, powered or not) and need to display the numbers on the bow along with the registration sticker. It is a bit of a PITA to file for a lost title and get a new one, takes time, a few bucks, the cops have to check the serial number, etc. but can be done. Without numbers or current reistraion sticker, the Park Rangers/Cops and/or Coast Guard hassle you and can ticket you if you are sailing on public waters. BTDT
In NJ you need a title for a Sunfish. If you get a boat from a state that does not require a title you must get a notarized bill of sale before they issue a title for you. Every state has different laws.