Thanks everybody - some really unusual ideas. I’m putting a page of them together, sticking to the modern materials at the moment. I’m tempted to have a go at building the plastic tarpaulin & willow wood kayak myself.
Check www.banburycanoe.org.uk and click the “weird boats” link.
Cement boats have been around for
several decades now, dating back at least to the 1940’s. The cement is a light material called ferrocement.