Hi there!
We can find the distance with this expression:
Height(top deck) - Height(bottom) = distance
The top deck has a height of positive 9 feet, while the bottom of the boat has a height of -24 feet since it is BELOW the surface of the lake.
Thus:
9 - (-24) =
Subtracting a negative becomes a plus, so:
9 + 24 = 33 feet.