I respectfully tip my hat to Bill as being the font of knowledge.
Font of knowledge? I think that's a polite way to say gun whore.
But anyway, I just checked Daryl's web site and saw that the gun ordinarily comes with a 5/16" shaft, so you could hardly go wrong with that. I have a 57" Magnum hybrid that has the track cut for a 5/16" shaft, but I use a 9/32" anyway so as to cut recoil to an absolute minimum.
However, the big fish our here, yellowtail and white sea bass, don't run into caves. The grouper where you will be diving do go into caves where they can bend the shaft against the coral, so you might be better off with the thicker shaft. Either way, you should probably bring at least one spare.
Which brings us to choice of tip. The gun comes with a choice of a Hawaiian (flopper) shaft or a threaded shaft. A slip tip for the threaded shaft is at extra cost. I'm a big fan of slip tips, but again that is a product of where I dive and what I shoot. White sea bass are very strong but have soft flesh, so a slip tip holds better.
On other boards where I hang out, I've read Florida guys saying that they like Hawaiian shafts because the shaft stays in the fish and gives them more leverage prying a grouper or snapper out of a hole. With a slip tip, the shaft pulls back out of the fish and only a steel or Spectra cable is though the fish, so you can't lever the fish around.
I hope this helps.
..all he has to do was to install mamba kits on his Cyrano 1100 and BAM!!! supergun..i can see the force is getting weaker