If I make the 4" pipe grid and it leaks, it will sink without warning, separate pipes will make it unsinkable - just like Titanic
It will at least give you warning by listing before sinking. Independent chambers is clearly the way to go.
When I was a kid, my grandfather owned a cabin on a lake in PA. A lot of people had "rafts", permanently anchored swimming platforms, made like a wooden deck supported by empty 55 gallon drums at each corner for flotation.
In Oregon, a 55 gallon drum with one head cut out sells for $15. The way the head is cut out does not interfere with the ability to buy another drum head and make an air and water tight barrel. Each drum provides about 400 lbs of flotation, which is about the best bang for the buck I can think of in that department.
The drums were not really attached to the barrels, the wooden structure had spaces to fit the drums in the substructure of the deck. We always pulled them out of the water before the lake froze over for the winter and re-launched in spring. They floated about 12" above the water as I remember, maybe a little more.
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Be careful here. A duck is the only creature on earth that can turn a pint of food into a quart of poop, or so it seems. Five ducks on a small pond is OK, but ducks show up from everywhere when the environment is good, and can turn a small pond into a manure pit in very little time.
This may lead to teaching your daughter the cycle of life, with you operating the shotgun...