Malibu is probably the closest area to Santa Monica for diving in terms of time or distance, probably 15 to 40 minutes drive time depending on where you are in Santa Monica, traffic due to time of day, and where you decide to go diving. There are some spectacularly nice dive spots for a shore dive, but as others have noted conditions can be crappy as well. We've had a lot of swell the past few weeks and the water has been a greenish brown, but the water is looking a lot nicer lately and conditions are relatively calm. In Santa Monica, I've heard the breakwater is diveable, but it's a long swim from shore and conditions can be rough. You won't get any kelp forest in Santa Monica. Down the coast from Santa Monica, Vets Park in Redondo is probably the next dive spot, but you won't get any kelp and it has limited structure.
Heading up the coast from Santa Monica, Topanga is supposed to be a nice dive with lots of reef structure to explore. Just past that is Tuna Beach, also with reef and some kelp. Malibu Divers is on PCH when you get to Las Flores. Before you rent any gear, I suggest you drive up the coast, check out conditions, then rent gear if it looks diveable to you. From Malibu Divers, you can be to several dive spots within 5-10 minutes - Malibu Road being the closest generally interesting dive spot
If you haven't done any shore dives with small to moderate surf, it's usually a good idea to go out with someone who can give you pointers the first time, both for surf entry/exit and beach diving in general.
North (actually more west than north) from Santa Monica, there is probably 35 miles of beach to explore for shore dives from Topanga to Deer Creek. Typically you won't get any deeper than about 35 feet. My favorite shore dive spots in the area are Escondido Beach, Leo Carrillo and Deer Creek (listed by closest to farthest from Santa Monica).
The water is starting to warm up, with water temperature around 55 to 57 now so you probably want a 7mm wetsuit (minimum) with hood and gloves. Dry suits (since your location lists Maine) are a hassle for shore dives, since they are bulkier getting in and out of the water, you'll need more weight, and are less streamlined for surface swims. For boat dives, diving dry is definitely nicer
I usually try to go shore diving most weekend mornings. Let me know when you're in the area and I can give you an update
Also as mentioned, there are a few six packs in Marina del Rey (Westbound, Giant Stride and the Goby), as well as plenty of dive boats that head out to the Channel Islands from Ventura (past Malibu)
If all this sounds like too much work, instead of just saying screw it, take the ferry to Catalina and just dive the Casino Point dive park for the day. Early in the morning, it should take only about 30-40 minutes to get from Santa Monica to Long Beach for the Catalina Express, the ride over is about an hour, and you can rent tanks and weights over there
If you decide to do a beach dive, make sure you have some place to secure electronic keys that come with rental cars! You'll want to lock your rental car and take the key with you, and a lot of the rental cars have keys on a wire loop with the remote (so you need something to cut the wire loop)
