Yes, BILLABONG is a Scrabble word!

Letter values in Scrabble US: B 3, I 1, L 1, L 1, A 1, B 3, O 1, N 1, G 2.

Is it a valid word?

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

a stagnant pool of water in the bed of a stream that flows intermittently
a branch of a river made by water flowing from the main stream only when the water level is high