The reason that all the squares got wet was because the factory just butted the ends of the squares next to each other...rather then leaving a gap and filling that gap with cabosil. The picture above is a very common sight on this board
Leave a space and fill the gap and that wont happen. Also it will be stronger.
As far as composites go...they are supposed to be waterproof and we know at least temporarily they are but I've seen water trapped in foam over time... they are expensive and you cant just go down to Home Depot and pick up a sheet for 30 bucks. Another thing to think about is that this is new technology that has not stood the test of time...whos to say that boats with composite coreing will not crumble apart in 30 years?
at least we know what plywood is capable of. That said I do like composits for some things and I have several sheets of corecell sitting in my shop waiting for something special to attach themselves to.
strick