Yes, HYDROFOIL is a Scrabble word!

Letter values in Scrabble US: H 4, Y 4, D 2, R 1, O 1, F 4, O 1, I 1, L 1.

Is it a valid word?

HYDROFOIL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a device consisting of a flat or curved piece (as a metal plate) so that its surface reacts to the water it is passing through
a speedboat that is equipped with winglike structures that lift it so that it skims the water at high speeds