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A man's personal, private parts - the genitalia, more specifically the penis. May also describe a condition of tumescence in said organ.
Bob's pants were ill-fitting, they compressed his chubmarine with unbearable pressure.

Jesse wished his girlfriend would pay more attention to his chubmarine when they attended the drive-in theater.
by kittywords June 16, 2009
Having a boner under-water.
While swimming, I saw a girl in a bikini and got a chubmarine.

alternate : Asian with a boner underwater is a "yellow chubmarine".
by Schmelvin Han May 01, 2013