Hi @Aquaman @MitchellMc
Firstly you have your reasons for a plywood pond base I get that.
But consider buying a plastic tub/ pond shape that you like it will be almost bomb proof for 40 + years if supported evenly. Drop it in ready to go and waterproofing method costs add up.
You can also buy QUALITY pond liner rubber/ plastic down side is foldind away excess material folds created depending on the shape. And the membrain is puncturable.
Never use Crommelin pond sealers out doors i used it once and not UV safe I went through hell grinding it of cement bricks to replace the coating as it cracked and pond was like a sieve.
First the plywood structure needs to be as ridge as you can get it as any water proofing can crack and fail with movement and over the joins and in the corners needs over lapping bandages / webbing to help reenforce those areas. Treat plywood both sides if outdoors for longer life.
The next thing is this out out doors as if is the plywood and frame work needs protection both sides even marine ply.
Options
Fiber glass and epoxy resins will seal nicly over webbing and ply but long term sun UV exposure will break it down over time (10 years) reepoxy required before break down to save heaps of sanding. Must acryilc paint will bond onto this but easy to scratch off. Has some flex.
Bitumous rubber paint will seal over plywood (always add webbing into the corners ) it can cover fiberglass and use it like a rubber mould build up the skin layers to 1 or 2 mm thick as a minimum. It may not bond 100% to epoxy or fiber glass but water pressure will always hold it in place. Effectivly it makes a water proof plastic bag with perfect fit to the pond design. Not paint able really and other products wont really stick to it. Also not 100% UV proof and may need re coating every 5 to 10 years where it is sun effected. highly flexable in case ply moves.
My favourite water proofing is cementous paint that is mixed with water proofing polymers that stops positive and negative water transfer through the cement paint. You do the same thing and webbing to joins and corners and use a stiff straw bristle brush and paint cemet on in layers 2 to 3 min. The last coats add oxide to colour the concrete if desireed or regular paint, will work or you can tile over the cement base and do any thing.
The finish is gritty like sand because it is made from sand but you can also finish with a smooth pure cement finish , And is sand able with grinder disc. !00% UV safe forever, Not really flexable. But if a settling crack appears simply web over it and re cementous paint a patch,
This used insde plywood would act as form work almost and will become very heavy so not movable later.
Depending what option suits you we can expand on details on specific products. All options fish safe once cured. Usually a tank will neeed to be filled then emptied after a week to wash off any lingering chemicals.
