You didn't say where you are going, but on Kauai, there are some rivers that flow into the ocean, and they are protected by sand banks, so sharks cannot get in there. The Hanalei River that flows into Hanalei Bay is one, and another one is just further down the road, and flows into the end of Lumahai Beach. You're actually on the beach, and the water at the end of the river is part ocean water, but it's absolutely shark-safe. I don't know about such places on other islands. Kauai has the most significant rivers in the Hawaiian islands, but I would guess you can find such places on the Big Island rainy side also.