Is there a reason why Balsa is not

Vinylester, though being a modified polyester, is still an ester resin; and it will generally absorb around 4% water in weight. I would use balsa or foam core only with epoxy, if only for laminate bonding strength. But this is only my opinion.

 

I used COOSA bluewater. It has roven/woven embedded on top and bottom. Prior to the installation of the COOSA I laminated 8 layers of alternation +-45 and 0/90 biax, then vacuum bagged the COOSA then went over it with another 6 layers of biax. The exterior was covered with a 20000K Carbon fiber laminate. The core sample taken out for the surface drive showed evidence of the benefit of vacuum bagging. MAS epoxy was used in all areas.

The hull bottom was using Airex Damage tolerant foam.

This was a 3 year project. The boat was originally an Intrepid 339 with an outboard transom. It was stretched to 36.5 feet to accommodate the twin Yanmar 6LY2 with 2 speed ZF gears and surface drives.

COOSA is the best solution for a waterproof super strong transom.