I've just left SD for Massachusetts, but sailed plenty while I was there.
Info on the Mission Bay beach bars is here: http://www.sandiego.gov/lifeguards/safety/mooring.shtml.
Most important note about the beach bars is that most are limited to boats 14 feet or less, so your 5.0 wouldn't be allowed. A few (but it's not clear which ones) allow boats up to 17 feet. I don't know anything about the safety of leaving a boat there.
You'll find an active multihull fleet at Mission Bay Yacht Club (www.mbyc.org
) on El Carmel Point. The racers are mostly on N20's, with some F18's and A's. There is also a diverse collection of Nacras, Prindles, and Hobies that sail recreationally.
I don't know of anyone regularly (or ever) launching out of Del Mar or Solana Beach. Not even sure where you would launch. The only ocean launch I know of is on La Jolla Shores (street parking only, and crowded in summer). Generally, boats are restricted from launching/beaching on ocean beaches in the area. Most boats launch out of the northwest part of Mission Bay and get to the ocean via the channel between the Mission Bay jetties.
Be proficient at raising and lowering rudders/boards...kelp will be something new for you.