After all sorts of issues with finding a contractor, I am finally back to a point where the work might start in a few weeks.
The pond is going to be quite large (T shaped, 24ft long, 18ft wide at the largest) and quite deep (sloped, 4 to 7 feet deep). I live in (lower) upstate NY, hence harsh winters, ice, snow, etc.
The pond will be surrounded by an 18" high wall, with coping on top. The pond walls will be made of cinder blocks filled with concrete, the bottom will made of concrete too, and the concrete covered with waterproof plaster, like pre-fiberglass pools. No gunite nor fiberglass will be used (too expensive for my budget). The water level will be ~6" under the coping, hence roughly one foot above ground level.
I was originally thinking that the top part of the walls would be covered with thin stones and hydraulic mortar. But this appears to be a rather expensive part of the equation. And we can't let the plaster as is in the dry space above water level, as far as I understand, it would crack. Also I've read all sorts of bad things about pond paint (e.g. Pond Armor), having to regularly redo it, etc.
Now I am considering what a neighbor did for her own koi pond, to cover the top part of the plaster with 2 feet (vertically, starting from the coping) of ThoroSeal. 6" of it would therefore be out of the water in the summer and I might (not sure yet) lower the water level by a foot or so at times (e.g. to empty and plug the jets pipes to prepare for the winter). Hence the 2ft goal.
As far as I understand, ThoroSeal can be mixed with some paint color (so that the aesthetics are ok with the coping) and provide excellent water/winter resistant properties. And it is also fairly cheap.
Thoughts? Feedback?