If you are going to use schedule 40 pvc and end caps, with purple primer and clear cement, I wouldn't worry about adding foam to the pipes. I also don't think I would go to the hassle of separate chambers. Just create a square out of four 90's and put some treated wood on top of it. Keep it Simple!!!
How often does schedule 40 fail when glued together properly under pressure? Rarely to never.
I've been thinking of the pros and cons of treated boards, like 5/4 decking with good sized spacing between them compared to a piece of treated plywood for the platform? If you have gaps, will that result in less debris on the platform?
Are you going to build a ramp that goes into the water so they can climb onto the platform?
Will you put nesting material on the platform or rely on them to bring it? Do ducks make nests or do they just flatten an area and use what's there?
Eddie