The sea lions basically took over a good amount of docks in a very expensive marina.
From Pier 39, you look down on the sea lions. You're fairly close so you'll get a good view of them.
Pier39.com answers the question if the sea lions are always present:
Yes and no. From late summer (late July) to late spring (mid May), there are typically hundreds of sea lions hauled out at K-Dock (ranging from 150 to 600+). In June and July, most of the sea lions head south to breeding grounds on the Channel Islands, although a handful to a few dozen have remained throughout the summer in recent years. In late July, non-breeding sub-adult males and juvenile females begin to migrate north again. Other breeding males travel north later, and some males migrate as far north as Alaska and British Columbia, Canada.
After seeing the sea lions, walk around Pier 39. It's a lively place with a lot of restaurants and shops.
It's usually always busy in this area. Parking can be hard. There is a public parking structure across the road from Pier 39. If you park there, take the elevator up to the 3rd floor. There's an elevated walkway you can take to cross the road to get over to the pier.
Don't expect parking to be cheap though.
Last Bit of Advice
There are pamphlets in the area, on the walls, that will explain when the sea lions got there. They'll also explain how you can tell the difference between a sea lion and seal. Here's a hint ... look for ears.