I was looking around for G10 in order to replace wood backing on my seacocks. I tried looking at McMaster-Carr but the costs were actually higher than what I got at Jamestown Distributor
. I really couldn't find it lower anywhere else for the size I wanted/needed.
As far as a core material, it is VERY heavy. Unless you were to replace a fairly small patch, I would think that it would be too much weight above the waterline that would affect how the boat sits in the water and its sailing characteristics.
Good luck on the repair, either way.