If you use the spray foam in the floor you are almost guarenteed to block off a way for the water in the front of the boat to escape.
Here's a tip on the "sheet" foam. If you find that the foam isn't the exact thickness you can take a wire (piano wire or something similar) and heat it with a torch and use the cross bracing as a guide and then "fillet" the excess foam off. You may have to heat it several times and when you feel it getting "stiff" or harder to pull through the foam just come straight up and use the last of the heat in the wire to cut out what you've already done. It works really well and you can go with a thinner guage aluminum sheeting for the flooring, reducing some weight. As for the sides between the braces, I have a great idea for that area but, I don't want to tell just yet. I want to get it done and then post pictures of it.