To "Cruise Line" is the act of pushing one's face in between two LARGE breasts, and rocking one's head from side to side very rapidly while making a vigorous, lip-vibrating "brrr" sound and in the process adding more power and more brr to the vibration due to the larger breast. It is recommend to use the "cruise line" on a girl with larger than a D-cup. Very close to motorboating just has to increase the scale of power.
A huge ship full of nightclubs, restaurants, pools, and staterooms with gorgeous views. Cruises are basically a free pass to hook up with a different person every night and party 24/7 without being judged. It's great because you'll never see the girl who held your hair as you puked your guts out that one night, or the guys who had to watch, ever again.
Clueless peasant: What did you do for spring break?
Tanned rich kid: I went on a cruise.
Clueless peasant: Didn't you get bored being on a cruise ship for a whole week?
Tanned rich kid: Let's see, I went to a different island every day, partied till 4 AM every night, and was handed everything on a silver platter... yeah, it was awful.