Yes, TIDEWATER is a Scrabble word!

Letter values in Scrabble US: T 1, I 1, D 2, E 1, W 4, A 1, T 1, E 1, R 1.

Is it a valid word?

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

low-lying coastal land drained by tidal streams
the coastal plain of the South: eastern parts of Virginia and North Carolina and South Carolina and Georgia

Also: tidewater region.