like the terms tits on a boar, man, or board etc... the term "tits on a boat" is used to describe how useless or incompetent someone or something is.

Believed origins of the term:

1) in history it use to be considered unlucky and a bad omen to have a woman on board a ship.
2) the figure heads on the front of a boat (usually a woman with baring breasts) though aesthetically pleasing had no real use other then for superstition and aesthetic.