I followed the first thread although I never posted in it and I got pretty excited when I saw the "revival" thread. But dude that is beyond what I imagined in every way as far as being amazing. That must have taken some serious time to set all of that up.
Also I have a suggestion for coating the inside to make it water proof. I think you should lay down plywood (or the marine grade plywood) like you were planning on doing and maybe fiberglassing the interior or at least just the corners. Then what would you think about using a product called Zavlar (Perma Dri in the United States)? Another member used it on one of his builds and it looks like it is very impressive.
Here's a video:
http://www.liquidrubber.ca/video/NewVideo.html
The last half of the video applies to this build more than the first half but the entire video shows how unique of a coating it is. The only downside is while submerged in water it turns into a dark brown color which is the only choice of color.
I think for a build this big a liquid rubber coating that can flex with the structure and withstand puncture attempts from large fish or decor would work perfectly. To be honest I know a thick pond liner is pretty rugged but personally I would always have that fear that it would take that one nick and it's all over. This is all just my opinion of course. Let me know what you think and good luck with the build.