possible, yes... practical, no...
A you already seem aware, cleaning a built in water 'tub' is going to be difficult at best. And since providing clean drinking water is essential, it may be more work than you are willing to do. I remove, clean and replace my Tegus water dish every 2~3 days. If it weren't such an easy task I would not do it as often as necessary and my tegu would suffer because of it...
This also limits future alterations. I built a nice custom filtered water container for my Tegu, then a few months later decided it was too small and too difficult to keep clean. So I simply pulled it out and put a larger plastic tub in. The newer plastic tub doesn't look nearly as nice, but works much better. If my custom water container was built in, I would be stuck with it and my Tegu would suffer because of it...
But having said all that, sure it's possible!
I've used Drylok to seal wood structures 100% water tight on several occasions. I used it to seal both my 4' x 2' x 2' Tegus grow out cage as well as his 8' x 3' x 3.5' indoor enclosure. Both enclosures are 100% water tight until water flows out the doors. I also used it to seal the custom filtered water container I referenced above, although it was not put to the test of time due to being replaced.