I'd avise driving down to Mission Bay to sail. Oceanside only has a ramp and dock for launching and no beach to stop at. Mission bay is surrounded by beaches. You can launch at several different places with sand next to the ramp to rig and set up a picnic. Santa Clara Point is the favorite cat sailing spot.
You can rent cats for several spots as well. North end of Santa Clara Point has the Mission Bay Sports Center. Campland also (used too, not sure). Camp land has easy launching and also offers mast up storage. You can also rent from Seaforth.
Places to sail Google map has lots of information. Zoom out then over to San diego.http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=117198149314367087564.000452e0a6a7a9350b790&ll=41.274968,-72.656965&spn=0.028705,0.053215&z=14