: do you have to put fibreglass sheet on the hull (inside and out) or
: can you just tape the seams and use a paint to seal the hull ...

Just tape the seams, inside and out.

: how thin can the plywood be if you can build a kayak like this (if
: it's possible).

3 mm.

My oldest kayak was built in the mid 1980s, still use it at times, 4 mm?, only glass on the seams. Second kayak, 1990(?), a hard 10 years of use, still used, 4 mm, no glass over, only glass on the seams. Third single is 10 years old with 3 mm ply. The kayak I'm usually paddling now is 2 years old, 3 mm ply, glass on seams. My partner's kayak, she built it over 10 years old, 4 mm ply, no glass except the seams. We paddle around rocky shores and the kayaks get hammered at times and survive very well. Day trips and multiday trips.

Alex