Manhattan Beach: Roundhouse Aquarium
There's a couple aquariums in the South Bay. They're rather small compared to maybe the Long Beach Aquarium or something of that space, but it's still fun to see the life that you can't usually see under the water.
The Roundhouse Aquarium in Manhattan Beach is one of my favorites just for the casual visit. It's free to the public with a suggested donation, and the location – at the end of the Manhattan Beach pier, is unbeatable. It's perfect to go and visit at the end of the day and check it out before watching the sunset.
I love the shark replica they put outside, and the "touch tank" where you can touch the starfish and sea anemones (they've apparently come up a lot in my life lately). And then there's a larger tank with some eels, and small sharks.
They also have a "kids area" upstairs for classes, and parties, and just has more educational things along with some varied tanks.
It's never a regrettable choice to visit.