Thanks to all who replied, it is very useful. I have been busy for the last few weeks, so wanted to collect my replies and gratitude in one reply for anyone who cares.
I found the Yostwerks transparent kayaks and may actually start there, as I know a retired carpenter who literally drools over the woodworks. I was going to use his garage as a workshop so want to keep him happy. Thanks!

As someone mentioned, PC does not scratch that easily and in addition, the difference in refractive index between PC and water is much small than the difference between PC and air, so any scratches tend to be less visible once submerged. In addition, it is quite strong relative to other materials, so I am hoping to keep the total weight down by using thinner plates.
It can also be shaped cold or with a minimum of heating to some extent, which is why I went for the Ganymede (all surfaces are relatively simple and the bottom is flat). This way I hope to avoid pre-drying of the starting plates, which would take a rather large oven...
The condensation issue I had not considered. I reckon it might be possible to surface treat the inside of the kayak with an anti-fog agent to suppress condensation. This happens to be related to my trade, so it should make an interesting challenge...
I appreciate that the majority of waters may have too murky water to see through. Admittedly, I just like the idea of having a see-through kayak for turning heads, so I am not too concerned about this on second thought. But thanks for letting me know what to expect and not expect.

I am of course happy to read any further comments that may come and will post links to any final build that may arise in due course.

Thanks,
Peter