Biggest thing spearfishing in Kauai Hawaii is respect, respect local fishermen, dive spots, and fish regulations and you well be welcomed with open arms.
Try to dive some spots around Princeville and found all the typical reef fish. Unfortunately most of the predatory fish here will have it to some degree, guys will go there whole lives eating anything they want and never have a problem.
Bring a thin suit and get ready to hold your breath for a while to be able to approach the reef fish. Try to dive right off the reef out front of Kapaa shores many times, also had good luck up at Larson’s beach as well.
Hawaii spearfishing regulations
Hawaii spearfishing regulations of fish has problems with cig so your best bet is to stay away form “hot” fish which include but are not limited to roi (peacock grouper), big ulua (GT), Kahala (amber jack), Uku (grey snapper or jobfish, though I eat them ALL the time, stay away from cooking the head or stomach), big Uhu (parrots), knifejaw.
One of spearfishing hawaii types are safe fish include smaller parrots (make sure they are legal, not sure what the new regs are but a google search should produce them for you), goats (kumu, munu, Moana ukalis, etc.), mu (big eye emperor, good luck), small jacks (papio, omilu, all under 30lbs or so) fish that are worth the risk. Uku, yellowspot papio.
Those parrots are smart, try to hook up on a local’s boat if you can to hit a place a little less visited. They have dive gear and might be able to point you in the right direction. The dive shop mentioned above has a list of divers looking to go out and their numbers.
If you are going to shoredive the typical spots, be prepared to be frustrated with fish that are extremely smart and cautious. Fillet most of my fish over 3-4lbs as from what you have heard (not a hole lot is known) most of the ciquaterra toxin concentrates in the head and stomach.
Behind Princeville there is a path that goes down to a private beach with an amazing reef and good fringe drop off’s. Probably one of the better places for being able to jump in off the beach anywhere and see fish.
Most of the spots there are winter spots so they wont be of help to you.The guys at Kapaa variety store are great for info on diving. The guy’s dove with regularly drop 70-80 feet and go in holes to bag big ulua and pull them out, those guys can dive.
There is also a full on dive shop scuba, freedive on the main road in Kapaa. Oh yeah pick up a ciguatera kit somewhere as the fish can be deadly. If you can make friends with a local and get tout to the tuna grounds on a boat, there are huge yellowfin, wahoo and mahe mahe depending on time of year. Good luck!!