For a $20 fee, Ames Farm Inn on Rt 11 in Alton (not Alton Bay), across from Diamond Isl., has 2 ramps with a dock between them and parking space for your tow vehicle & trailer on the premises. That's my regular launch site for my 25 footer. They're about 9 miles (about 15 min drive) from the public docks in Alton Bay up Rt 11.
Also, like sa meredith said, make sure you have a current chart and make sure you use it!!