There is a ramp right at the end of A1A where the ferry crosses at Mayport. You can launch there and motor out to the ocean through the jetties. Sailing near the mouth of the St. Johns is difficult due to the current. Most of the sailing and racing is done in the southern part of the St. Johns, south of the city of Jacksonville.
There aren't many trimamrans in Jax. We have one at the Rudder Club and ther is an F24 (I think) at Whitney's Sail Center, south of the Buckman Bridge in Orange Park.
http://www.rudderclub.com