It may make you feel reassured to know that this has been successfully tried.
Check row 5, columns 2 and 3 on this page:
http://homepage.mac.com/masonmarsh/kayak/PhotoAlbum19.html

The guy has finished the kayak and no news of his drowning so far...

: Is it advised to fiberglass the interior of plywood hulls? My next two kayaks
: will have plywood 'stitch & glue' hulls and I am trying to minimize
: expense due to high cost of importing epoxy and fiberglass.

: I recall that Chesapeake Light Craft only glassed the cockpit area and taped
: the interior bow and stern seams, but some others (One Ocean Kayaks) glass
: the whole inside.

: My kayaks will receive moderately rough use - occasional rough surf landings
: and handling on-shore but mostly tranquil soft use.

: I think I can get either 3m or 4mm marine ply to work with, so am thinking of
: going 4mm with only taping the inside except for the 'butt to toe' cockpit
: area.

: Advice would be appreciated!

: Thanks in advance,

: John (building in Fiji)