Something similar to what you’re describing is being pursued by the National Aquarium in Baltimore, whose staff are planning to relocate all the dolphins there to a sanctuary in Florida. Among the sites under consideration are a number of abandoned quarries next to the ocean. Check out the post here, where you can see an example of this. It includes a PBS Newshour video in which science reporter Miles O’Brien joins the National Aquarium’s CEO, John Racanelli, as they scout out some promising locations. The fact that they’re looking at seaside, not inland, quarries is important, since these connect to the ocean, and you’ll see that they’re full of ocean flora and fauna. An inland lake, even filled with salt water, would not easily provide an environment that’s close to what the whales would experience in the ocean.