Dr. G, your posts beg the question of why you are using such inferior materials to build your drone boat? If, for example, you were to use corrugated plastic sheet instead (a material commonly used here in the States for cheap signage), you'd have a two ply material that was weld-bonded to it's internal webs, relatively easy to cut, especially with a heated blade or wire, but otherwise tough in the projected environment. Painted to protect from UV damage, it would be very durable. I used a 4 foot by 3 foot section, unpainted, for about three years as a sun-shade for a window mounted air conditioner unit before it started 'falling apart'. Painted, it might have lasted forever. Bends along the axis of the webs is no problem - can even be used as a hinge. One more thought: with the open edges of the 'web' sealed or filled with polyurethane foam, you have built in bouyancy.
This is one link for a supplier from a Google search, just so you can see what I'm talking about
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Advantages of Coroplast Corrugated Plastic Containers | Coroplast
Cheers!