Green’s Dictionary of Slang

little man (in the boat) n.

1. the navel.

1937–84

Partridge DSUE (1984) 689: late C.19–20.

2. the clitoris .

1890–1904		Farmer & Henley Sl. and Its Analogues.
1949		Monteleone Criminal Sl. (rev. edn) 236: the little man in the boat The clitoris.
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		C. Bukowski Erections, Ejaculations etc. 31: The top of my head bumped The Man in the Boat and then Sarah would [...] moan.
1980		Maledicta IV:2 (Winter) 1834: Some terms that have survived this ignorance include little man in the boat (or boy in the boat), little ploughman.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
1988		J. McDonald Dict. of Obscenity etc.
1999		D. Lypchuk ‘A dirty little story’ in eye mag. 8 July 🌐 After fondling her flapjacks and kissing her Aunt Nelly, he found the man in the boat.
2001		J. Stahl Plainclothes Naked (2002) 103: The clit seemed less like a little man in a boat than Don King on a yacht.
2003		Simon & Burns ‘Ebb Tide’ Wire ser. 2 ep. 1 [TV script] The girls always telling me about the little man in the boat. Now I know who they’re talking about.

In phrases

sink the little man (in the boat) (v.)

of a man, to have sexual intercourse.

2003

phreakin ‘Phreakin Featuring DJ Dizzy (San Diego)’ posting 5 Sept. at TheScene.com.au 🌐 it’s about trying to sink the little man in the boat! Phreakin all night long!