Trust me , you will not be able to use your flyfishing gear on Maui.
If you go to Lahaina, and the Northwestern Part of the Island, you may be able to catch a few beautiful catches, but your line will get caught in the rocks.
I would also suggest trying SCUBA DIVING or Snorkeling @ Molokini Crater.
Not that many rivers, if you wanted to go fly fishing, you could leave Kahului airport, one morning, and fly over to Lihue on Kauai, go 15km North of the Airport, until you hit the Wailua River. Excellent place to try flyfishing.
BTW: Many Films where filmed on the Wailua River, as well I heard you need saltwater gear.
IF you have any msgs PM me