for fast casual, IMO you can't beat Paia Fish Market. One in Lahaina and one
in Paia (I just noticed Kihei too) . Every bit as good as places where we've spent 3x the money, including Mama's Fish House.
If you want to try Mama's, make a reservation now, they may be weeks or months out. Mama's does have a very nice setting, if that's important to you.
I've heard the Lahaina location is the tastiest, but I have not tried the others. In Lahaina they're on Front street near the banyan tree.