I recently purchased a kayak, just for scuba diving. I am just getting it dialed in. There is no reason you can not dive around Monterey off a kayak. It is a great way to get out past the kelp too, since it will slide over the top.
One of the main reasons I purchased a kayak, was because my boat diving experience in Monterey has been terrible. The Monterey Express last time, left me very discouraged (I think they tried to kill us!). Many of the places the dive boats go, a kayak could easily get to. For instance, in front of the aquarium, which is a nice dive spot. You will see all kinds of kayakers in rental setups out touring around, but not diving there.
I am looking forward to going to back to Lobos with my kayak. Last time we dove Bluefish cove, we had a great dive off my buddies Whaler. It would be no problem anchoring a kayak out there in 40 feet of water.