I've started giving some thought to foam vs airtight compartments vs compartments with breather holes and inspection hatches. The designer recommends two part foam, and many on the Bateau forums have used it successfully, but there's also some horror stories out there, often (not always) involving water getting in through unsealed holes and penetrations.
My thoughts:
I don't want fully foamed compartments below the waterline. I'd always be worried about water getting in there even if it is unlikely. Neither do I want nothing but airspaces. I'd be concerned about water getting in there and pressure from the air expanding and compressing with temperature changes working at weakpoints in the structure. Also unlikely but...
On the other hand hatches and limber holes can let water in as easily as out. In the worse case of a swamping or some kind of structural damage to the hull the boat could go under very fast without some flotation.
So far this is what I'm settling on. I'll have several compartments in the hull under the floor. I'll use some of my already bought and glued together EVA foam pads (about 150L) to fill some of this, and some medium density pour foam (65kg/cubic metre) to fill strips near the stringers of the central underfloor compartment, supporting the sole and leaving the central area free for ventilation. Bung at the back of the central compartment, small plugs for the side compartments, with 2mm breather holes up at the bow end to equalise pressure. I'll use a hole saw to allow air to flow through the frames where needed.
Above floor flotation will be down the side walls above the floor level so I can get my toes under, and also in semi watertight storage seats etc. Looking at about 300L of foam buoyancy which should be enough since the hull material floats, even with fibreglass skins. I just need to float the motor, battery and crew.