The act of swaffling (smacking your penis against something) fast and furiously between a woman's breasts. Similar to motorboating but with a penile element.
You could motorboat your girlfriend, but may derive more pleasure from pontooning her instead. Just make sure it is ok to ejaculate in her cleavage before doing so.
Jennifer: My girlfriend and I tried pontooning last night!
Debra: Oh my goodness you gotta boat?!
Jennifer: Noo no no, we both took Viagra and watched our shit swell up, it was like two huge fat pontoon boats!