Getting a blow job while in a hot tub, or any other form of water. To be defined as a blowfish, the girl's head must be completely submurged under water, except to come up for air of course.
Girl: Wow this hot tub is nice.
Blowee: Yeah it is, have you ever given a blowfish?
Girl: No, what's that?
Blowee: I'll show you, here's a snorkle.