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. [late 19C–1930s] the navel.

2. [late 19C+] the clitoris .

In phrases

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

[1990s+] of a man, to have sexual intercourse.