Type V Portland
cement is better than the more common type III. It is much more resistant to saltwater (akaline) than the type III. The commonest mistake is adding too much water
to make the mix more workable, this results in a low strenght concrete with a lot of surface problems. The stiffer the better. Adding PVA to the mix makes it bond better.
Putting it in the water right after setting up is a good idea as it prevents any surface cracking due to drying out (evaporation not curing). I am not so sure about the salt
though, some type of barrier would be an idea to prevent salt
from leaching in to the surface layers. You may want to slap on multiple coats of PVA before in goes into the water. It drys fast as the water in it will be absorbed by the concrete. One source of cheap
PVA is primer paint
for sealing new drywall, it comes in 5 gallon buckets (it should say PVA on the label).
I must add I am not a ferrocement boat person, I have just been around a lot of concrete.