The California Channel Islands are a fantastic dive destination. As i see it there are two main options diving either the Southern Channel Islands out of Long Beach or San Pedro or the Northern Islands out of Venture or Santa Barbra.
Southern Islands include: Catalina, San Clemente, (my favorite), Santa Barbara and San Nicolas but this island is very weather dependent.
Northern Channel Islands: Anacapa, Santa Cruz, Santa Rosa and San Miguel Island.
Usually local dive shops will charter boats and you can rent tank and weights with them. A really goos book with 98 dive spot descriptions is "A Scuba Diver's Guide to the California Channel Islands" By Dale and Kim Sheckler a resource I would highly recommend.
Water is temperate a 7mm wetsuit with hood or even better and what i use is a drysuit. You can stay in Venture or Santa Barbra dive the Northern Islands and do a side trip up to Monetary do dive that area as well.
I have few California scuba videos on my YouTube channel - Peter Neubauer I live in San Diego so i am close enough to have dove all the islands. Kelp forests are a great ecosystem to explore.
Hope this helps