They are pretty heavy depending on the mix design you use for the concrete. Our university takes part in an annual concrete kayak/canoe build-off with after varsities. The thing is that you nbeed to put some reinforcing in the boat. You can play around with concrete strengths by using hardners and fibres in the concrete. The one thing is that because concrete tends to be more riggid than fibreglass, they can crack easy.
I will see iof I go some stuff on the boast that we built.
The construction of tthe boat is challenge though and great fun.