First of all, if you're going to bury a highway, bury the Gardiner, and built a high capacity subway alongside it running the entire length of the lake. It would connect Union, the Ontario Line's ex station, and provide robust public transit along a major East West route that doesn't have it, both out to the beaches and to south Etobicoke.
You could then sell literally all the land that the Gardiner is on for development to pay for it.
Why would you stack another highway, under our existing highway? And why would you potentially build an East West train literally next to the Eglinton LRT?
The tunnel will have to be waterproof no matter where we build it, and there are lots of tunnels in the world directly under rivers / lakes, it's one of the main reasons to build a tunnel.
But the city does not have the necessary infrastructure to deal with the excess water. You can water proof it all ya want but if you can't get rid of the water it builds up and ya got flooding