A few points to toss in he

Boats with significant amounts water ballast (i.e. Mac) need much less
flotation because the specific gravity is so close to one. In other
words, a tank of water only needs flotation for the tank material, not
the contents. A number of the smaller water ballast boats have
flotation, Catalina chose not to on the ground that it have taken too
much space.

An other way to look at it is that you only need significant amounts
of flotation for the heavy parts of the boat, like the keel and
engine. This is why my catamaran can have positive flotation, in the
form of six watertight compartments. I'm told that the hull by itself
(which has foam core above the waterline) has a SG of under one.

Its easy to do a "napkin calculation" of how much foam is needed: Say
a 36 foot boat displaces 12000 pounds. That's 333 pounds per foot,
we'll make that 500 pounds because of the pointy ends. Since water is
about 62 pounds per cubic foot, that's 8 cubic feet worth. If we then
consider the circumference, assuming a 12 foot beam, is about 36 feet,
what we're left with is an average coating of under 2.5 inches. In
other words, if the hull were constructed with a sandwich containing 3
inches of foam, it would be enough to provide positive flotation. It
would also give a huge amount of stiffness, collision protection and
insulation.

We can also roughly figure the size of airbag flotation - 12000 pounds
is about 200 cubic feet of water, which is a cube about 6 feet on a
side, or probably better as two spheres of about a bit under 6 feet in
diameter. This should be quite doable.

One problem with these calculations is that they assume a typical
racer/cruiser, not a heavy displacement offshore passage maker.