Yes, WATERMARK is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, T 1, E 1, R 1, M 3, A 1, R 1, K 5.

Is it a valid word?

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

a line marking the level reached by a body of water
a distinguishing mark impressed on paper during manufacture; visible when paper is held up to the light

Also: water line.