On these size boats with light skins, the core densities are the key to dent resistant hulls.

With a standard glass skin, about 100kg/cubic meter and you should be able to drop your knee on it without a mark. About 80kg/cubic meter is border-line and commonly used for racing boats (because dents don't really matter for performance), 60kg/cubic meter will dent just looking at it and is only really suitable if you don't care about dents but care about weight.

No core, no dents.

If you're planning to run your boat up the beach, consider a metal strip down the keel. Easiest solution, sand will always win over composite.

Never build a balsa core below the waterline. Despite what the suppliers say or how good the boat is built, it will eventually get water into the core and it will be the beginning of the end for your boat. Above the waterline balsa installations are fine and very good for dent resistance and other properties designers want out of composite cores.