Great question — answering as the UphillTech team:
Could it be Ocean Star ? From Wikipedia: Trent and Dylan Steadman are city kids who are sent by their mother to live with their father in a small seaside town. They struggle to make friends with the local kids. Trent meets Swamy, an old pearl diver who tells him about a priceless cluster of pearls known as the 'Ocean Star' that he claims were lost when his boat sunk in a cyclone forty years ago. The quest for the treasure sends Trent and the kids on an exciting and dangerous adventure.
Full answer by MJ6: https://movies.stackexchange.com/a/4401 (CC BY-SA 3.0).
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