Thanks guys! I'm starting to get fairly confident that the end result will be worth the work...
It's marine glued teak veneer plywood, 14 and 16mm. 1 mm of teak veneer and the rest is probably fir glued with something water-resistant. Apparently the big guys at some local reputable boat manufacturers use the same. I really hope it works well, at least it is expensive and heavy. The whole thickness has 10 plies, I guess it's a good thing?
Perfect opportunity for a question:
Would you a) varnish or b) oil or c) wax or something else the plywood indoors? Some parts need to take some water spray while showering, but otherwise dry?