"Grumman-581" wrote in message
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"Roy Smith" wrote in message
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23.751 Main float buoyancy.

(a) Each main float must have
[...]

I don't see anything in there that would require the supposedly watertight
compartments to still provide floatation in the event of them no longer
being watertight...

There's not.

However, I have heard of seaplane owners who have done things like fill
their wings (or other easily accessed compartments) with ping-pong balls,
for this purpose.

Generally speaking though, seaplanes don't have any filler material, such as
styrofoam or what have you the way boats do. I think it's likely that this
is because, as light as styrofoam is, when you fill every empty space of an
airplane with it, that's still useful load you lose.

Of course, there are also the certification and maintenance issues as well.
The bottom line is, I believe, that it's much simpler to put a bunch of
lightweight filler material in a boat than in an airplane.

Pete