On one hand, I've read about swimmers getting bitten by sealions, and they're certainly intimidating, especially when they swim rapidly under your kayak and make contact with the bottom of the hull.
On the other hand, I recently saw a video of a scuba diver who went diving with them! The sealions looked like they were trying to tear him apart, but he was fine, he explained, "Sea lions are like dogs, they don't have hands, they ask questions with their teeth. So when you see 4 sea lions all teething my flipper, although it looks bad, in fact it isnt, they're just intelligent and curious animals who want to investigate something they're never seen before, I had to hold onto my mask when they started teething it, of course, but these are huge, powerfull animals that could have torn me limb from limb in an instant, nevertheless, I was fine, no damage to me or me gear at all".
I think they're intelligent enough that you might have certain individuals who develop aggressive behaviour patterns just like certain people do, but in general they're more curious than hostile.
Still, it's easy to be intimidated by them, isn't it? I know I am.