the most popular fishing port is probably Kona on the big island but you can catch ahi in way more places than hawaii. in Kona they do things different and you can get a boat for cheap $300/day but they wont allow you to keep your fish..... thats what they did with me on this ahi and the guy had said nothing at all about not being able to keep fish. i didnt want to kill any billfish but wanted to keep any ono (wahoo) or mahi's or tuna. but i was not expecting him to say that the fish was his and that a buyer would be at the dock when we got back. if i am going to be a commercial fisherman, i want to get paid for it. not pay to do it.
I figured that the charters were a lil cheaper because the fishing grounds are right out of port and you can do alot of live bait fishing wich is fairly easy on the fuel. i would rec omend that if your going to go on a tuna vacation to try mexico or costa rica. Hawaii has the best chances of a "grander" but for a better chance of a hook up i would go elsewhere (unless its june/july in hawaii... then go to hawaii)
for the money...... sailfish can be alot more exciting. try Quepos in Costa Rica and most boats average 10-15 sails/day around January
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"time is but the stream I go a-fishing in"- Henry David Thoreau