Yes, SEAHORSE is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, A 1, H 4, O 1, R 1, S 1, E 1.

Is it a valid word?

SEAHORSE is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

either of two large northern marine mammals having ivory tusks and tough hide over thick blubber
small fish with horse-like heads bent sharply downward and curled tails; swim in upright position

Also: sea horse.