The difference would be similar to the problem with corrosion of iron reinforcement, as we see in Americas' concrete structures that deteriorate rapidly, vs the use of pozzolanic ash, as was done in ancient Greece. Greek Concrete has survived the ages, weathered and worn, but not as easily damaged, either.
The more caustic environment of Geopolymer vs Portland Cement will inherently provide superior protection of ferrous reinforcement, which, currently, is considerable more budget-conscious than Composite reinforcement.
That said, IMHO, the combination of Basalt composite reinforcement and geopolymer cement has the additional benefit of zero corrosion and the ease of repair, given that geopolymers will adhere to geopolymers, while Portland cement based mixtures rarely adhere well.
In addition, the lower ion transmission of geopolymer will aid in the reduction, or even prevention of bi-metallic corrosion, which can be induced simply by having an electrical shore connection, and grounded metals in the hull structure, using the water to complete the circuit.
Another benefit is heat tolerance. Portland Cement based concrete dissociates and has spalling, when exposed to fire, where Geopolymers, not being bound with water, do not have that problem. The difference is in the chemistry. While I'm an amateur, if I don't understand the equations, I can still understand the descriptions of the differences, as well as knowing real-world problems with concrete.
IMHO, the inherent properties of the combination of Basalt reinforced Geopolymer far exceed the properties of ferrocement, in too many areas to ignore. By having access to a suitably low-cost geopolymer, the added expense for higher-priced Basalt reinforcement will be mitigated in the durability and ease of repair.
If I can get the mixture ratio and technique simplified, the finished cost may equate to current ferrocement construction, using similar techniques, meaning it is also a familiar process.
Just on my current basic formula, I'm already below Portland Cement based mixtures in cost. If added expense of Basalt reinforcement raises it slightly above ferrocement, for a vastly superior product, then it's a win/win situation.