My experience is that Catalina has much better conditions than on the mainland. I did have good luck shore diving out of La Jolla but that's almost San Diego and likely we hit it right/got lucky. In general boat diving will have better vis vs. shore diving.
You need an instructor so while your looking for one ask for recommendations on where to go and what vis is like. CA is notorious for low vis shore dives The dive park in Catalina would be ideal but its pricy to go there, but very nice. $100 a night minimum and $150-$200 may be more inline. The Ferry over/back is approx $100 per person. However its likely cheaper than Disney!
We spent Friday night on the Great Escape and motored over to Catalina Saturday. That was a three tank dive and not really designed for classes. But there may be an OW instructor doing classes off a boat. There are a lot of dive boats out of LA/Ventura. I am unsure what snorkeling would be like on the mainland. Catalina would be interesting for snorkelers. There were a couple dudes spearing and free diving off our boat in Catalina. We only spent one night on the Great Escape but they were headed back to Catalina again on Sunday and you can stay the night on the boat if diving. We did a live abroad out of Ventura (The Peace Boat) and there were students on that boat. A 7mm wetsuit will be required.
I love the diving in CA but I am used to 3' vis, cold water, and a lot of gear/weight. CA is great but more advanced than say Florida.
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Sorry, you asked about vis and water temp. Obviously varies, but in summer vis off islands was 20' to 70', and close to shore San Diego boats and La Jolla shore dives vis was 5'-25'. I have dove off most of the islands and vis can be excellent. We had very bad vis a couple days on the boat out of San Diego and very good vis at La Jolla. Water temps vary but 45F-65F depending on depth and if there was an upwelling. So 7mm territory or Drysuits. Tropical would be a better place for a 10 year old unless she loves being in water regardless off temp. I swam in CA water when I was a kid.