The picture below shows the core material we used in the infusion kayak. The hex shapes provide the separation between the carbon layers. The spaces between them lets the resin flow. What you are looking at is the inside of the cockpit section of the deck that gets cut out after cure. It only has an outer carbon and one core layer to give the resin a path to flow. There are actually two different core materials shown. The smaller hexed pattern was used on the deck and the bigger hexes in the hull.

I would have thought this would give a heavy boat but we were able to produce a 22lb (fully out fitted) kayak vs a 15lb hand lay-up.