ok, besides being cool gadgets heres my thoughts on it:
1 - On a size less then 100 you really have to be super sensitive to feel the difference in maneuverability underwater.... all my -100 guns are aluminum
2 - Besides being lighter its also stiffer..what does that mean, no barrel flex..do we ever power a gun to feel that benefit, probably not..except on a long gun once again, for example a 130 rob allen..you could be subject to some barrel flex with the alu models
3 - Anyone that owned an alu gun for many years, knows that at some point water gets in (probably from reef/rock abuse and sometimes its just a stupid o ring that needs to be changed)..and that could be the end of it..with carbin, that doesnt happen..its not a hollow barrel..
4 - Can carbon break..sure..but really how many posts do we have here of someone breaking a carbon gun..
5 - of course design is more important, but for the same superior design, in certain applications, carbon could be more superioer then say aluminum...
6 - every application/hunting condition will play a role in the best material suited...although ive seen blue water carbon guns, i dont imagine that they could ever be superior to wood for that application, the mass and resistance to recoil of the wood should deem it more appropriate for blue water/multi banded guns...
ok i think im out of more points for now

cheers