It's hard to guarantee sea lion sightings. Your best odds are at certain sites that are either a bit far (Coronados islands just south of the border, or the rookery at Santa Barbara Island, which is far enough that you generally need an overnight boat), or sites that may be a little advanced for you given your lack of cold water experience (oil rigs.) For maximum safety and enjoyment, I recommend taking the ferry to Catalina and renting gear hiring a guide from Catalina Divers Supply. The dive park there is beautiful; it's a marine preserve so there's tons of life. There are stairs to make it easy to get in and out, and there are nearby benches to gear up so you won't have to walk far with all that heavy stuff. The guides are excellent; they will help you figure out your weighting needs with those thick 7mm suits, explain how to deal with kelp, and show you some cool stuff. The town of Avalon is adorable, and it's a great day trip.
ETA: I can definitely promise kelp in the dive park at Catalina in June. Based on my experiences, I'd give you about 5-10% odds per dive of a sea lion sighting there, which is better than you'd get just about anywhere on the mainland.