Where in Hawaii are you? If your near me I would love to take you out.
In my opinion (and many others) the best length gun for Hawaii spearing reef spearing is 90-110 cm. there are many options but most people start with either a Buchat Marlin, Picasso Basik or Century, or a HammerHead. I have never owned a production gun but many of my friends are big fans of the century.
Most spearo's here like open muzzles and pined shafts. both are modifications that you can get done at Hanapa`a or any other dive shop. I think a century with all these mods costs about $300. You don't want to be spending much less than this on a spear gun you'll be better off saving up with a 3 prong until you have enough money saved up.
As for choosing length of your gun 90 cm is most popular with divers new to guns but many wish they had longer gun in a short amount of time. i would talk to the guys at Hanapa`a and see what they have to say.
I would recommend going to Hanapa`a instead of Maui Sporting Goods (if you are on oahu) Hanapa`a has better service,prices and more selection. Also the guys there sometimes give you little stuff for free.
I've never been to Hawaii but I get the impression that something like a 100/110cm railgun (Rob Allen/Rabitech/Hammerhead/Pelaj/Mako/Freedivers/Orcas) or european speargun (Omer/Beuchat/Sporasub/...) is pretty normal. Hawaiian rigging is sometimes a little different though - open muzzles seem unusually popular (probably because of Hammerhead) but it has started a worldwide trend - and double 14mm bands (which is almost unheard of here) or single 16/18/19/20mm bands. Your choice of band(s) should take into account your strength and loading technique as well as power/range - I find 1x16mm more than enough power for most of my needs - 20mm can be a bear to load.
The fact that you have seen the fish that you wish to spear is useful because estimating their distance will likely be a key factor in choosing the barrel/spear length. Maybe you need a 120/130/140/150? The size of the fish you are after is another factor -- there are some big fish in Hawaii and the railguns are typically rigged to handle very big fish - upto and beyond 200lb+ (significantly thicker spearline, stronger clips and swivels) although most european spearguns should handle quite large fish (maybe 30-70+lb?).
Right you are Mr. X although many divers switch out there 14mm bands for 16mm when they improve.
Just for fun I'll give a little recap of what is popular in Hawaii just for
Blaiz, feel free to give your 2 cents
for euro or rail guns people like open muzzles and pinned shafts, for easy reloading. on euro guns people usually have a glue on shaft guide, this along with the open muzzle is said to improve accuracy.
Most people use 9/32 inch (7mm) shafts with a hangover of about 12 inches (30cm). we like the thicker shafts because of the wide range of fish being targeted on any given dive. you could shoot a <1lb menpachi and then, 5 minutes latter shoot a 30+ lb Ulua (Trevally). You gotta be prepared for anything.
Most guys either use 2, 9/16 inch (14mm) bands or 2, 5/8 (16mm) bands with. I have never seen any Hawaii Spearos use anything but circular bands. And almost everyone uses thick cord wishbones.
Also the most popular lengths are 90-110 cm as they match the visibility and target fishes well.
Also Mr. X there are many Hawaiian spear gun makers, as you mentioned Wong
Wongspearguns.com
and HammerHead
Home of HammerHead Spearguns
but there are others, some of the biger companies are aimrite and hatch
Aimrite Spearfishing
Home - Hatch Custom Spear Guns
those are the big ones that you can get out of state.