Dry for wet is a film technique in which smoke, colored filters, and/or lighting effects are used to simulate a character being underwater while filming on a dry stage. Fans and slow motion can be used to make hair or clothing appear to float in the current. In recent years, it has become possible to digitally add rising bubbles in post-production, heightening the realism.
The technique was pioneered by Georges Méliès who would use a painted backdrop to suggest an underwater environment. By the mid-20th century, it had become a reliable technique that was used extensively in productions like 20,000 Leagues Under the Sea and Voyage to the Bottom of the Sea.[1]
[The Shape of Water is] a 19.3 million dollar movie. I couldn't do tank work, and I remembered—I used it once on Hellboy—an old theatrical technique called dry for wet, and I decided that's what I'm going to do. I'm going to puppeteer everything on the screen—wires for everything, open ceiling on the set—and I'm going to have ten to twelve puppeteers moving the objects in front of the camera.