His grades were below sea level
The son answers, "Y." "Because I want to know!"
2nd Monster: He's at medical school. 1st Monster: Oh what's he studying 2nd Monster: Nothing they're studying him!
The third grade.
Who gives a cluck (I wonder if the moderators will censor this joke merely on the grounds that it is categorically terrible )
Shoot him before he hits the ground
One can't see to go the other can't go to sea.
They are both always surrounded by sea - men! I made up this one.