I put a lot of thought into this, as I really don't want to be ripping off the sole and digging out sodden foam ten years down the road, nor do I want to end up a statistic.
Standard 2 part polyurethane foam, that I bought from an industrial supplier in ChCh. The blocks are high density foam 100kg/cubic metre and the pour foam was partly the same high density and mostly a standard 33kg foam. All supposed to be closed cell.
I am aware of the potential for it to absorb water. Apparently this is mainly due to not being mixed perfectly with a 1:1 ratio, at the right temp and humidity, or a bit of water gets in through where the denser skin has been cut and then through hydraulic action, slamming of the boat, or freeze thaw cyclesmigrates its way through over years.
Otherwise it is pretty waterproof, especially the polyurethane block foam made in a factory at optimum conditions which is therefore of higher quality.
Thinking about all this the key in my mind is to a) do a good job of laying the foam b) keep water from sitting in there and c) allow a method of checking and removing water from compartments. The foam is there to allow me to sleep better at night without that nagging what if...
The middle compartment is by far the biggest. I glued two layers of high density block foam down the middle of it as a dam. This is quite strong; a small square will support my weight. Then high density 2 part pour on the inside of them to help secure and support.
Then multiple pours of the 2 part standard density foam between that and the stringers. That explains all the little muffin tops. One or two places where the 2 part didn't set off correctly, which I will mostly dig out and replace. Once I've cut it level I'll go about plugging and sealing any little places where water can gather. I'll keep the muffin tops for later if they are good.
Outside compartments have the block foam just glued to the floor with some high density pour foam on the upper side between foam and chine. The high density takes longer to go off; I underestimated how far it would migrate. It does do a good job of plugging holes and water traps.
Each side has two separate compartments, the middle just one big one. In either case the lowest part of every single compartment is left bare and uncovered. Should water ever get inside in any significant amount it will end up there and I will know about it.
When the sole goes on it will be epoxy coated and glassed both sides and glassed in all around the perimeter as per the plans. There will be five inspection ports. Circular screw in type. To avoid leaks through these they will not be in places where water can gather. Two at the back will be partly up the vertical motorwell bulkhead, the other three will all be on the sole, but either inside a hatched compartment or at least under a shelf and on a raised surface. I'll seal them as well as I can, but no water should ever be sitting up against the inspection ports. There will be some small breather holes but not where water should get in, and if it does, I will know.
The cockpit will be designed so that if and when rainwater gets in, it will run to the rear and either out the back or out two plugholes I'm putting in the sides. No nooks and crannies for it to accumulate. Benches will have either a sloped uphill side or gaps under to let water by, either salt, rain or when I hose it out.
The boat will be stored outside but under a cover. I may get one custom made eventually. Thinking about a carport as well to keep UV and the worst of the rain off.
If water ever gets into that foam I will be surprised.
There will be foam under the gunwales as well for a measure of upright buoyancy. Not yet 100% sure what exactly it will look like.
As you can see I didn't just come up with this plan over a couple of beers.