if that's 3/4" ply switch it for 1/2", that's all you need and plenty strong enough. if you have 3/4 switching to 1/2 will save you a good 10-15 lbs of weight.
on mine I got the closed cell Styrofoam sheeting they put on houses and layered it under the floor gluing it together with spray adheasive. its going to make it a lot quieter so you put less sound through the hull into the water, plus its an extra safety feature and adds floatation if the plywood is screwed down or if not it still is something to hold onto if you ever sink but wont help you draft less water.
on my boat I had a sheet of aluminum riveted on top of the foam board so I never have to replace the wood anymore.
for more floatation (higher draft) you need to add floatation pods to the back to raise it out of the water more.
you need something like this:
http://www.cabelas.com/product/Beave...ods/847206.uts