Dont buy a gun for a weeks diving in Hawaii, buy a gun for what you will be diving the most. When you get to hawaii you can always get a cheap polespear, which many of the good divers here use (most good reef fish are found in holes and caves and therefore very close contact, polespears can actually be more productive than guns. Multiple shots on the same drop, and sometimes multiple fish)
If you choose to get a shorter gun, say a 55 or 75, they will work in Hawaii. You may not be able to hunt the deeperwater species but seeing as you dive in a lake, you are probably accustomed to shallower water.(I mean this in the nicest way:blackeye) A 90cm gun will be a great gun for diving in Hawaii but make sure it will be ideal for where you dive.
As far as gun type, Like IL Diver said Omer and Hammerhead are great guns. Both probably priced fairly similar except HHs are set up with an open muzzle and 2 bands(little more power, longer to load) Both these guns are popular in Hawaii but most opt for the open muzzle. Also Beuchat, Sporosub, Rabtech, Rob Allen.... all great guns. I personally think that the only "real" difference between these guns are the price. Its true that "off the shelf" each gun will shoot a little different, but with modifications you can make any gun suitable for your type of diving.
I have heard that JBLs are popular for hunting carp but thats really the only good thing I have ever heard about them. I think they are like a shotgun, great at close range but blow a huge hole and from far... couldnt hit the broad side of a barn.
Good luck