Despite several snickers above about the cabin boy's letching after the captain's daughter, it seems to me probable that he'd never seen her.

So why was the captain's promise motivating? Because marriage to the captain's daughter--with a dowry, without a doubt--would lift the cabin boy out of his lowly class and change his life. That could be enough to make even a known homely spinster seem attractive.

Reminds me of the Gilbert and Sullivan lyric about the old barrister's daughter, whom he was trying to marry off:

She may very well pass for forty-three....
In the dusk, with the light behind her!

Dave Oesterreich