Tracing leaks

With the advent of digital cameras, you have instant access to pictures. If you can take a laptop to the boat, you even get a blown-up view immediately. That allows you to at least inspect the interior of compartments which have hatches.
Dry newspaper is useful for detecting where pinhole leaks are if you can get it to lie flat on the bottom panels.
If there is water in the other compartments, perhaps placing the boat on trestles and looking for water leaking OUT of the boat may help (haven't tried this myself). If you do find such spots, check for signs of rot there too, as the water is likely to have penetrated the centre layer of the plywood too.
Examples of interior pictures:
Under rear seat / transom

Starboard floatation tank interior

(Note: I've never done any of this, but it's what I would try doing.)

Maybe you could use pressure to detect air leaks. You would need to have some kind of pump and feed the air into the tank through one of the drainage holes, blocking it off so that air doesn't escape round the outside of the tube from the pump: there is a putty that can be used for that, as used in composite manufacturing when applying a vacuum to a part to infuse epoxy through it. You wouldn't need to put the pressure up much at all, and indeed shouldn't do so as it would damage the hull if it was to go too high. Any holes in the tank that let water in when sailing will certainly let air leak out. When mending bicycle inner tubes, sometimes you can't hear the hiss of escaping air unless the hole is pointed directly at your ear from an inch away, which is why a basin of water sometimes has to be used. Putting the boat underwater for this and looking for bubbles wouldn't be practical, but perhaps the liquid given to children for for blowing bubbles could be brushed onto the hull and would form bubbles wherever there's a leak.

Once you've identified the locations of all the leaks, each one could be filled by angling the hull to get the hole/crack at the lowest point, then put some freshly mixed epoxy (Araldite would do) over that part (on the inside of the tank - throw a small ball into the tank and it should settle at the lowest point, telling you where to put your little pool of epoxy) and leave it to run into the hole/crack, perhaps helping it through by pumping a little air into the tank as before. If epoxy comes out on the outside, let the air pressure in the tank back down to normal, remove what's left of the pool of epoxy if you're sure there's only one exit point for that hole/crack, and keep wiping away any epoxy that leaks out. This will ensure that once it's solidified (after an hour or so) it will have sealed it all the way through, thus protecting the wood from further damage that would occur if water was still able to get half way in.

It would be worth checking for leaks above the waterline too, and on the deck, because water will get in much more easily through a hole in the bottom if there's also a hole at the top for air to escape through.

I'm guessing that if there are no inspection hatches, mending the holes would normally be done by stripping tape off the outside of the seams and replacing it, followed by a lot of repainting. A less disruptive alternative to that might be to use a vacuum to reduce the pressure in the tank a little, then it could suck epoxy in through the hole or crack from the outside. Not ideal, but it could be a quick fix that might last for years. It's probably harder to get hold of an inexpensive vacuum pump though, and again it would be essential not to overdo the pressure difference as it could destroy the hull if it goes too far. A subtle difference of one or two percent is all I have in mind.

I have used the air pressure method with great success on my daughters grp concept 302, only use a low pressure !. HTH