Thanks Duncan!
Ferrocement boats are built and work, but are nigh impossible to re-seal once they get cracked by an impact with a jetty etc. At least not in a way that won't open up again when flexing in the waves.
And chloride corrosion of the mesh is a big problem in seawater.
I'm hoping I won't need a perfect seal and that I can keep well away from seawater!
Will see...