What sits at the bottom of the sea and sings "Give me the Moonlight"?

Q: What sits at the bottom of the sea and sings "Give me the Moonlight"?
A: Frankie Prawn.

To understand this joke, the listener has to know about the song "Give me the Moonlight" - specifically, they have to know that it was sung by Frankie Vaughan.

The humor in this joke is derived from the fact that the joke references a famous person, but mispronounces their name in such a way as to make it a possible answer to the question.

I doubt that many millennials would get this joke. I just told it to 3, and 0 of them did, so probably you shouldn't tell it unless you are among people of the baby boomer generation or older.