Just to add a word or two to Bill's comment ... most of my encounters with sea lions have been anywhere from neutral (brief swimbys) to playful (juveniles at Santa Barbara Island). Once I was even drawn into a game of keep-away with a piece of yellow caution tape the sea lions had found (they swam it by and dropped it near me so that I'd pick it up and swim off with it).
Once, though, I was buzzed near the park boundary at Casino Point on Catalina by a sea lion that seemed more like it was staking out its turf -- circled by us, blowing bubbles at us and showing teeth. Made it hard to lay the tape measure on the sand floor for our navigation class. ;-)
I also recently spoke with a DM for an L.A.-area dive shop who said she was given a couple of hostile bumps by a large sea lion at Santa Barbara Island. She suspected that this was a mom protecting a small offspring that the diver had gotten too close to.
Also on the marine mammal front, back around March several divers on a boat I was on had an interesting interaction with a large harbor seal at one of the coves toward the west end of Catalina. It came up within a foot or so of one diver group and stopped; "It seemed as though it wanted its tummy rubbed, so I did, and it seemed to like it," one diver told me. The same harbor seal abruptly came up from underneath and appeared out of nowhere, its face up about a foot from my face, as I was freediving in a few feet of water (that was a memorable shock!). My first impulse was to bring my camera up to my face and press the shutter button; apparently none too thrilled with the strobes, it swam off, not to be seen again on that visit.
