Green’s Dictionary of Slang

little man (in the boat) n.

also little boy in the boat, little old man in the boat, man in the boat
[note little man n. (1)]

1. the navel.

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

2. the clitoris .

[UK]Farmer & Henley Sl. and Its Analogues.
[US]Monteleone Criminal Sl. (rev. edn) 236: the little man in the boat The clitoris.
[US]Trimble 5000 Adult Sex Words and Phrases.
[US]C. Bukowski Erections, Ejaculations etc. 31: The top of my head bumped The Man in the Boat and then Sarah would [...] moan.
[US]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.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
[UK]J. McDonald Dict. of Obscenity etc.
[US]D. Lypchuk ‘A dirty little story’ in eye mag. 8 July [Internet] After fondling her flapjacks and kissing her Aunt Nelly, he found the man in the boat.
[US]J. Stahl Plainclothes Naked (2002) 103: The clit seemed less like a little man in a boat than Don King on a yacht.
[US]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.

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