It's obviously cold - this time of year probably a full 5-7mm wetsuit - in winter many dive dry. You can rent wetsuits at a nearby shop. Those would include In2Deep Diving or Eco Divers. Or Malibu Divers on the way to Ventura but they may not be open that early. You may want to rent what you need the night before.
I don't think there's a diveboat in Santa Monica any longer but there's 4 good choices in Ventura about 90 mins north.It's not such a big deal along the coast there but traffic, not distance is a factor. If your friend lives there, they'll know.
Explorer Diving Aventures - Home Page
https://www.peaceboat.com/Default.asp
Welcome to the Raptor Dive Charters Website located in Ventura Harbor, California
The Spectre Dive Boat | Diving Santa Cruz Islands on Spectre Dive Boat
They're going to dive the Channel Islands - for Sea Lions you want Anacapa.
Fast, Fun Action at Anacapa’s Sea Lion Beach - California Diving News
I think they all go out there but Spectre advertises it on their homepage. All should have a trip calendar also.
Typically the DM stays on the boat. So if you're by yourself - all will have rental guide options. Also they provide different levels of gear since many in SoCal bring their own.
hth, I haven't been out there in a while so verify everything I posted with the shops/boats.
Santa Monica is nice. Some years ago they converted a street downtown into a pedestrian mall and there's some good restaurants, shops etc. There's also
Santa Monica Pier