Yes, WATERLOOS is a Scrabble word!

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

Is it a valid word?

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

a town in central Belgium where in 1815 Napoleon met his final defeat
a final crushing defeat
the battle on 18 June 1815 in which Prussian and British forces under Blucher and the Duke of Wellington routed the French forces under Napoleon

Also: battle of waterloo.