[alexrom1207]

I can't give you answers to all of your questions but I can help you out on a few.
1. Even in warm water it is important to dive with a wetsuit if you're going to be in the water for a long time. Even 80 or 85 degree water can lower your body temperature significantly if you're submerged long enough. Not sure exactly how long "long enough" is, but you'd probably be best off getting a thin shorty wetsuit (like surfers wear). This way you will only need minimal weight to obtain neutral buoyancy.
2. Fishing with a hawaiian sling is more difficult than fishing with a gun. Those members who've started out using Hawaiian slings and have become proficient at it say that once they switched to guns they found it much easier. But they were happy they started with the slings because all in all they're better spearfishermen because of it.
3. Teak seems to be the preferred material for handles as it is easier to work with than aluminum. --> this is just from reading other threads as I have never made a Hawaiian sling myself.

Anyway, good luck with the project. If you check out the DIY section you may find some good info on how to make your sling. Or even just pictures of what it may look like.