Yes, MAHIMAHI is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, H 4, I 1, M 3, A 1, H 4, I 1.

Is it a valid word?

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

the lean flesh of a saltwater fish found in warm waters (especially in Hawaii)
large slender food and game fish widely distributed in warm seas (especially around Hawaii)