Florida(or Hawaiian) sling shafts to avoid? to choose?

Some background.
Ive never spearfished before. Ill be diving in the gulf of Mexico from Sarasota county shores, occasionally with a kayak.
I may go from a boat once in a while, but I doubt Ill shoot anything big if I do.
I have a ~30" draw length.

Im thinking 9/32nds(simply because its between 1/4 and 5/16 and I want to be able to take medium-sized fish.) but have no clue if thats a good choice.
I was also thinking of 56"-60" because Ill be diving in shallow water taking shots near the bottom and possibly reefs.

On to the questions...
What diameter would you use and why?
What length would you use and why?
Any brands / diameters / lengths to avoid?

Whatever shaft I get I will attempt to attach a shooting line to. (Somewhere forward of the sling at full draw.)

Thanks much.

P.S. I want to use a Flopper and / or slip-tip.

Quote from Jareyberrysplash

if you've never spearfished before I'd steer clear of the Hawaiian sling altogether and get a pole spear or a speargun. However if your set on using a sling I like rob allen shafts. They come in metric sizes, 6.6 or 7mm. I personally use the 170 cm 6.6mm shafts and have yet to bend them even after I've seen some decent sized grouper drag then under rocks. Also its not really possible to put a sliptip on a sling shaft. Also please be careful and find someone experienced to dive with spearfishing is a dangerous sport and I've seen some of my friends get hurt because they got in over their heads.

Thank you. I have read about the pros and cons of slings / poles and guns already. Originally I was going to go with a pole spear, but after much deliberation I chose a sling W/ a shooting line. I shoot a bow and arrow and the action of a sling seems more natural to me than loading a pole spear. Thats not the only reason however.
6.6MM or 7MM sounds good.
Ive had bad experiences in other aspects of life by moving too fast. This is a scenario where moving too fast will potentially kill me(unlike other scenarios...), so Ill be sure to move slowly. Thank you though.

Quote from Jareyberrysplash

I had a similar experience when I first started using a sling and now I hardly ever use a polespear. ...SNIP... Don't get me wrong I don't think it would be impossible for a beginner to learn to spearfish with a sling just more difficult.

Understandable. I suppose I should have put that "I plan on using a shooting line" in the first post.
Ive hunted on land with an atlatl and a bow and arrow. Im all for a challenge. However, Im all for not-losing a 35 dollar shaft when a fish runs.

Quote from Wishihadgills

Just about every dive shop I have been to recommended starting on a sling. I personally hated it but it was a good way to get used to hunting and it does make you have to get very close witch helps later on. the other reason a sling is great is you can get a block and a cheap shaft for 50-75 $ where as a decent pole spear is at least 150$. I would recommend 9/32 or the 7mm (almost the same) and after seeing Dan's method for attaching line to it I wouldn't do it any other way.

Thank you. I plan on attaching a line like Dan showed in the past. I still have a few details that need to be worked out. All those details require me to have the items in my hands to rig them up, so thats for a future time.
Sounds like Ill be going with a 6.6 or 7mm.

And as always, "Thanks Dan." It seems like you have good advice in every thread.
Thats not to say the others who responded dont, its just that I havent noticed enough of their posts to say one way or the other.

I used my Frost Mora knife to carve my Atlatl out of a piece of wood left over from home construction. I believe the piece of wood was a furring strip. It took about one and a half hours to carve.
It took a little while longer to figure out what to use as a nock, maybe another couple of hours of looking through stuff in my garage. Im not sure what it is I used, but Ill get a pic of it tomorrow.
I made the shaft narrow enough that there is noticeable flex when you swing it. (Adding to the elastic force imparted against the arrow.)

Using an Atlatl is a lot of fun. I enjoy the motion of throwing a spear more than using a bow and arrow.
Hunting with a bow is easier though, because you dont need to make a big movement to shoot a bow and arrow, as compared to the movements to throw a spear.

I havent killed anything with an Atlatl in Florida, because Florida says its not legal to use.
If you read any dictionary Ive seen or ask any college Ive asked, they all say technically an Atlatl is a bow, but Florida has a complicated definition of a bow, making an Atlatl illegal.
Im trying to get that changed.

Ive killed two rabbits, one squirrel and a bird.

The rabbits I was ~15feet from. Both of the rabbits knew me because every morning I would sit in my backyard and watch the sunrise while the rabbits were eating grass ETC. After a while the rabbits started to trust me.(Eating near my feet, turning their backs to me, running up to me when they first see me.) After about 4-5 months I noticed that the rabbits had a litter.(I noticed this because their den was in my side lot thats all woods. I can see the bunny trails in all the underbrush.) About 1-2 months after the litter I killed the mother. Most of the babies survived.

The squirrel used to scare birds from my backyard and make loud noises while eating. I had my friend take my dog to my backyard from the north side, I went to the south side. The dog, who was ready to chase the squirrel, caught the squirrels attention enough that the squirrel didnt notice me sneak up behind it / wind up and throw.

The bird was attacked by a hawk in my backyard. It could not fly. The hawk flew off without killing the bird. I went out side and put it out of its misery.

All animals were consumed.