I know this is Beervana, and I know that there's probably not great interest in Hawaiian beer. That said, my travels were instructive enough that I think there's something worth bringing back to the mainland.
I managed to track down beers from three different breweries while in Hawaii--from hometown Kona where we stayed, from Mehana, a brewery across the island in Hilo, and from Keoki, from Kauai. I didn't get to try Maui Brewing's beer, mainly because they were $17 a sixer. (Everything's more expensive in Hawaii, so I had gotten used to $10, but seventeen--ouch!)
Leaving Kona aside (I'll come back to it in a minute), the two smaller breweries had a distinctive quality in common that I have to contribute to the water. I tried four of the Mehana beers and one of Keoki's, and the quality was the same--an unusually acidic, quasi-Belgian quality. It was as if they were hyper-carbonated, but not fizzy. It harmonized most delightfully with the Mehana pale and porter. Both could have passed for Belgians; the former seemed a little like an abbey ale (though lighter), and the later was Belgianish, but uniquely so. Instead of finishing creamily and sweetly, they had a sharp, tingling tartness.
When I took a coffee tour, the guide said that the soil is acidic, and later, when we visited the volcano, I saw why--the blasted landscape is scrubbed by intense acids that can devastate plantlife merely by wafting by. The tap water is full of minerals and is slightly sour. Oregon is famous for having soft, neutral water that contributes very little to the beer (but doesn't interfere with the malt and hops), so I am a little unused to sussing out this quality. But boy, is it obvious in these beers.
Kona, weirdly, is a standard-tasting West-Coast style brewery. I suspect they treat the water to address this, but I can't confirm it. Their beers taste much like an Oregon brewery.
If you make it to the Islands, definitely try the local beer--it's an education.