[Marwan]

Ole, some things here still dont add up:
1 - if you want a big gun to handle sharks, it may be an inappropriate gun for the actual spearfishing (too powerful). You need to know more about the conditions and the species you will be after and this will determine the best gun..
2 - Power heads are avaialable commercially in the US, they are classified as personal protection devices..the casing is sold commercially, bullets that go in it are not im afraid..and you also have to check the local laws there to ensure that its acceptable
3 - SHarkshield will probably be the better bet, all members will need to wear one, the whole odea of fending off sharks for the rest of the group is not very practical in my opinion...shark shields are expensive though...and NO you cant have one and huddle closely to eachother...
4 - Using a GPS with water proof casing, towing gear and swimming this long distance is not very practical, as you can see too many stuf to deal with if anything goes wrong that you have to deal with and say drop the GPS, the water or any of the vital items, this could be fatal...
5 - You really have to get some spearfishing and freediving practice before you do this, i dont know how recent you did your 10km swim and if the others of your team are as capable or have done it before, but it seems that 1 and a half months is a very little window to get these right...
6 - If the temp is between 24 to 27, then maybe 1.5mm wetsuit, i think a 3mm will be a lot for that and you may over heat, but then again i never tried being in the water for 8 hours so dont know for sure..
7 - Theres a thin line between wreckless and irressponsible, nobody is calling you names i think, just trying to talk some sense into you, since you seem to be leading others in this, you should know what youre up against, so that you dont feel sorry later..i ask you to please look at this as experience from others which you should use to your advantage.. You swam that 10K before with a life vest and a boat in your tow for emergencies, now you will be with a fellow diver either wearing or towing weight belts (which you need for spearfishing), along with other stuff, like float, fish, water, snacks, spare gear

Again please consider kayaking at least, its not easy by any means over that distance and still requires physical training as well as the freediving and spearfishing training..but you get to reduce some of the risks at least..maybe aim for that and if it gets too easy after a while try for the swim, at least by then you would have known the area better and increased a lot of the skills and probably narrowed down your packing requirements...i really do not recommend your original plan and find it quite dangerous for you and your friends, thats quite different from adventurous, and NO not all adventures need to involve this degree of risk..