Re: Kona fishing
They were probably chasing papio, juvenile Trevally. "Whipping" is what they call it down here, not a whole lot to it. First, Get yourself a medium action 8-10' rod, spinning reel with 10-20lb line. Rig either a plastic bubble float or a 1/2-1oz. egg sinker on your mainline behind a swivel, then between 2-6' of light mono or fluorocarbon with a size 2 "AH" hook. Favorite baits are small curly tail grubs, glitter strips, or cut pieces of squid. Good colors are white, black, or "firecracker". Whip it out around good looking structure and rip it back, the faster the better. If you're using the bubble float, you want it to be splashing and kicking up a fuss, with the egg sinker setup twitch it back with a fast retrieve.
Alternatively, you can do away with the float/sinker, and tie a small kastmaster to the end of your leader, fished with the same fast twitchy retrieve.
All kinds of reef fish will go after it, papio, barracuda, snappers, trumpetfish, etc. If the folks you saw were fishing in a shallow protected harbor or river mouth, they may be after small Akule, a small mackerel that makes great bait offshore or slide baiting for bigger fish. Same setup for them, just downsize your hooks and baits.
Fish any tide at first light, later on look for a rising tide, the closer to high slack the better. At night, put a glow in the dark grub on and try rocky areas for Menpachi, a tasty little red squirrel fish.
If you're going to be on the island for any length of time, I'd highly recommend heading to the library and checking out the "Fishing Hawaii Style" series of books by Jim Rizzutto (sp?), they've got most everything you need to know about shoreline fishing in the islands.
Hope this helped, good luck out there, and can't wait to see the results!