Quentin doesn't want to break into SeaWorld—it's not like they're filming Blackfish 2 or anything—but they drive out there and break in anyway. It's as easy as running through the moat that surrounds it.
After Quentin makes it across, Margo follows… and a snake bites her ankle.
Quentin leaps into action, sucking her ankle to get the venom out, but Margo notices that it's just a garter snake.
They climb a fence and boom, they're in SeaWorld.
Literally in the next paragraph they get caught, but Margo bribes the guard with a hundred-dollar bill to let them go, and he does.
They fox-trot together around the seal tank and do one quick dance before leaving the park. It's Margo's way of making up to Quentin for once rejecting him at the sixth-grade dance.