It means the activity of putting one's face between a woman's breasts, and rocking turning one's head rapidly from side to side while making a noise like a motorboat.
"Put your head between her tits and go bbl bbl bbl bbl bbl..."
The placement of one's face, specifically the mouth, into the area between a well-endowed woman's breasts, followed by a rapid shaking of the face in a side-to-side motion accompanied by yelling. The resulting sound that is created sounds similar to an outboard boat motor.
Sherry was wearing a low-cut top and Mike approached her and started motorboating in her cleavage.
A pornographic act describing getting a blowjob from a girl with pigtails - holding the pigtails in each hand and pulling her head in so that she gargles on your penis, making a sound like a motorbike.