Your carbon fibre
mast is not metal, so it will not matter if you use copper or aluminium on it. Since you are concerned over weight, perhaps the alloy would
work, but I would be using copper in your situation all the way, it is heavier, but the copper discharge plate in the
water will keep itself free of most
marine growth.
If you wish to use alloy, that will
work too. I would certainly run an additional lightning conductor and not use the carbon fibres in the mast as a conductor.
The whole idea of lightning protection, is not to collect and discharge a major strike. There is no lightning protection system that would do that--I have seen a multi-conductor lead sheathed one thousand pairs of twenty pounds per mile telephone cable completely vaporised and absorbed into the surrounding soil by what was only a moderately powerful strike, and a cow that had been grazing a couple of feet above it was also dead without a mark on it, so it must have provided some sort of shunt pathway as well.
The idea of a lightning protection system that works (and most of them do to some extent) is to conduct to earth any possible electron pathway of ionised charged moisture, dust and
salt laden air that may potentially provide a strike discharge pathway to your building or vessel at sea--and the ocean is a much better sink, being an electrolyte, than is the earth in most places.
In the same area as where the cow was killed, I later installed gaseous arrestors in a metal cabinet where open aerial suspension cable was used because of rock outcrops that made buried cable impractical. It was a known lightning hot spot, and after that was done one could often hear the crackle of these low pressure tubes discharging to earth the accumulated static charge of each protected circuit.
Which is why I think a gaseous arrester would work connecting copper lightning protection in the water to an aluminium mast--as soon as the charge potential reaches about one hundred volts or less, we used 100 volts because the circuit voltage ring and
alarm circuits were eighty volts--the self-triggering arrester fires a bit like the tube in an electronic flash unit, and continued to conduct until the charge potential is down to about eighty volts or so.