A poster above suggested thinking of the 2-liter bottles as other than
a flotation force.

I agree. What about cutting off the top and bottom, then cutting
down the length to create a plastic rectangle (albeit one that wants
to roll back up!), and use these as a hull material
overlaid/glued/sealed together as one would shingles on a roof. This
could be done over a very light "baidarka"-like frame (see baidarka
articles in WoodenBoat). There have been articles in Mother Earth
news about people that have done this with endless piles of beer cans
to literally shingle a roof with aluminum -- beer cans with ends cut
off and opened up to their rectangular panel.

It may take a lot of beer to actually contemplate such a thing, but
its been done. So why not with plastic for a simple boat. Wow! It
would be equivalent to a glass-bottomed craft for a great underwater
view!