Channel Islands Trip
Sorry for the delay in responding. I tried a couple of times last week, but the site kept crashing. Anyway, the islands you want to visit will have an impact on the charter companies you can use. If you want to go to San Miguel, Santa Rosa or even the west end of Santa Cruz Island, I suggest you leave from Santa Barbara. The only real option there is the Santa Barbara Sailing Center. They have Catalinas in the 28-50 foot range. The 4th of July will be busy, so you should check with Sherry at the Sailing Center (800-350-9090) to see what boats they have left. If you want to visit Anacapa or the rest of Santa Cruz, Ventura or Channel Islands (Oxnard) harbors are good jumping off points. Marina Sailing has a Catalinas and Hunters in Channel Islands harbor, and I think Pacific Sailing is still operating in Ventura, but I am not familiar with their fleet. I would not sail out of Marina del Rey unless you want to go to Catalina, in which case there are better/closer chartering options in Long Beach and Kings Harbor.
If you give me an idea of what kind of trip you are looking for, along with the number of days you have and the crew's relative experience level, I would be happy to suggest itineraries, hikes, good sea caves for exploring and like. (I am not above living/sailing vicariously through others when I am stuck on land). Although the closest and most popular anchorages will be crowded over the 4th, there are plenty of other great anchorages to explore, particularly if you don't mind a little longer sail.
Feel free to email me directly if you prefer.