Yes, WATERLINE is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, T 1, E 1, R 1, L 1, I 1, N 1, E 1.

Is it a valid word?

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

a line corresponding to the surface of the water when the vessel is afloat on an even keel; often painted on the hull of a ship

Also: water line, water level.