Re: bottles
I have never cared for the "flotation bottle" design. Before you do this, take a bottle that you plan to use. Fill with air, screw on the cap then start submerging in the water. Notice at what depth it crushes. It does not take much depth, just a few feet to start to compress the bottles. The more the bottles compress, the less buoyancy they have, and the boat will sink more, causing the bottles to compress more, and it spirals down from there.
Make sure you are aware that the design you come up with will actually work.
They will compress some, but think of it this way, even compressed to half their size, that bottle will still contain more trapped air or buoyancy than a chunk of foam of equal size, even not compressed. Then take into account that the foam may one day be water logged to some degree, the bottles seem the way to go.
One concern is longevity, especially with soda bottles as they don't hold up well in sunlight, but if sealed in the floor, the UV factor is eliminated. Just make sure they can't move around and rattle or chafe.
Those oil bottles seem ideal, but I'd stack them opposite each other so as to get as many in there as I could. A dab of glue or 5200 under each one and i between them would hold them in place for eternity.
A buddy uses bottles to float his pool cover, and to hold the edges down over the winter. Soda bottles do fine under the cover, but not around the outside, they last only a few months and crack, but I get him transmission fluid jugs that have lasted many years outside all winter. He's got nearly enough soda bottles to completely cover the surface of the pool, he tosses them in, slides the cover over, and puts a few sheets of carpeted plywood over the pool near the gate entrance just in case someone wanders up there and falls in. Those bottles held this winter under the 3+ foot of snow we got, the cover held, and the bottles kept it all afloat with no damage. I'm sure the pool froze but the bottles survived the winter in all that.
The one thing that bottles won't do is add to hull rigidity like foam does. Many boats use the foam filled lower areas to reinforce the outer fiberglass to some degree, loose bottles won't do that very well.
For this reason, I'd limit the use of bottles only to my aluminum boats, but pink insulation seems to be just as good a solution for those for me.
You could do bottles and foam, to both save on foam and retain some of the original structural support.
