The USS Bowfin is a submarine from WWII, nicknamed the "Pearl Harbor Avenger" since it sank 44 enemy ships. As a note, the USS Bowfin and USS Missouri are NOT National Park run (even though they are in the same location), so there are entry fees for these. They are run by preservation groups, so the money you spend goes into preserving these wonderful monuments to history.
Anyway, you can tour it (self-guided) separately for $10, or you can get a combo pass for a self-guided tours of the Bowfin and the USS Missouri for $26 ($16 for kids). We opted for both. We also opted for the headsets that explained the tour, which are HIGHLY recommended, as there are no signs explaining the same things, so you learn almost nothing without them.
BIG NOTE: Kids under 4 are NOT allowed on the submarine. Otherwise, the last tour leaves at 4:30pm and the place closes at 5pm.
If you get to Pearl Harbor early enough, you DO have time to do all three vessels (Arizona, Bowfin, and Missouri), and it's worth it to do all three. The symbolism is really neat: where WWII started, a vessel that did a bit of the fighting, and where it ended. It's a good history lesson.
To get to the sub, go RIGHT (as you're looking at the water) from the parking lot at Pearl Harbor. You'll see it, no shuttle necessary. Pay for your tour (or a museum visit) and go aboard. You can see crew quarters, fore and aft torpedo rooms, the periscope, engineering, mess deck, and other compartments. Allow about an hour, more if you spend a bit of time in the museum. It wasn't crowded at all when we were there—it might depend on the day—but probably most folks just want to see the Arizona Memorial. Why come all that way and NOT see other important WWII crafts? They are REAL, not imitations.