My plan would be to get all the foam in there you can and then buy a cover for the boat or put it inside 99% of the water that gets in a boat comes from the sky not the lake.
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What a simple and novel idea! Take care of your boat and it will last and stay looking nicer longer. Covering a boat to protect it from the elements is something everyone should do.
As far as drainage under the foam, it's a waste of time and money. Once the water has seeped down from the top of the foam down to the hull, the foam is ruined, saturated, and worthless.
As for the installation of expanding foam, installing foam after the decking as already been permanently installed is a bad build. This method will leave voids and craters that will hold water. Pouring the foam before the decking is installed will be your best build, this way you can get a complete pour without voids and craters to hold water and you know exactly what you have. Another bad thing about pouring the foam after the deck has been installed is that the foam will stick to the bottom of the decking, and if the bottom of the decking isn't sealed this will telegraph any moisture from the decking into the foam and vice versa.
Providing drainage below deck is important, it should be a drainway at the keel running fore to aft about 12" or more in width. These are easy to make, you can either block them out before pouring the foam or cut and remove the foam after it has been installed. A keel drain serves 2 purposes, one is obviously for drainage and the other is for ballast to keep the boat from capsizing should it take on water on the deck. The water on the deck will go into the bilge drainway via an installed deck drain and hold in the drainway to be pumped out with the bilge pump.
:facepalm:
