The island itself is always off limits. You are not allowed closer than 300 yards to shore anywhere. But you are allowed outside of the 300 yard limit in zones "B" and "C". You are right in advising extreme caution to anyone contemplating a visit. This one is not for the faint of heart. The weather in that part of the Channel Islands can be downright nasty.
I once knew an old time abalone diver who used to work that area. He told me some very interesting stories about San Nicholas, but I was not into shipwrecks at the time. I wish I had been.