Concrete-hulled ships certainly aren't unheard of (I've even actually helped work on building one).
Weren't there concrete+straw barge-boats made during WW2?
And I also seem to remember reading about massive ice & straw container ships, perhaps in WW1 maybe even through until WW2.
You're thinking of Pykrete (aircraft carriers) thought up by Geoffrey Pyke. in the 2nd WW.
The late great (Dr?) Magnus Pyke is/was a 1st cousin of Geoffrey Pyke btw.