Wet for Dry

Wet-for-dry is a term in the film industry for a special effect wherein an actor or prop is filmed in a water-filled tank, then imposed onto the film most often via Chroma Key or similar technology. The purpose of this is either to facilitate Slow Motion or to create the image of a supernatural creature not entirely bound by gravity. Filming in water works most effectively on hair and other long, flexible appendages, so expect this effect to take full advantage of such.

The opposite of wet-for-dry is dry-for-wet, where a subject filmed on a stage is imposed onto a water backdrop to avoid having to film underwater, thus making it possible for actors to do a scene while also being able to, for instance, breathe.

Examples of Wet for Dry include:

Examples of Wet-For-Dry

Multimedia

Clouds (by means of dye solutions) were frequently done this way before CGI, though in a sense it's all still wet.

Film

The dance of the Delphi in 300.
The Dementors in the Harry Potter films. Also, in the Half-Blood Prince film, actress Georgina Leonidas (who played curse-victim Katie Bell) was filmed wet-for-dry for the scene in which she is suspended in the air, so that her hair would look like it was flaring around her head.
The Ghost of Christmas Past in the Muppet Christmas Carol.
The spirits that come out of the Ark in Raiders of the Lost Ark were filmed in a tank to get a ghostly flowing effect on their hair and clothes.

Live-Action TV

The Doctor Who episode "The Satan Pit" had a Wet-For-Dry as one of the characters floated dead in space.

Music

Wet-for-dry was used in a Music Video for Portishead.

Examples of Dry-For-Wet

Film

One of the oldest examples is Twenty Thousand Leagues Under the Sea. There are scenes with actual divers, yes, but other scenes requiring James Mason and Paul Lukas to be seen through the helmets were accomplished this way.
Sam's near-drowning scene at the end of The Fellowship of the Ring. Also, the drowned ghosts in The Two Towers, those which weren't CGI.
Nearly all the underwater scenes in the film version of The Hunt for Red October were actually done with models in a tank filled with air and smoke, because the smoke created the "just murky enough" effect the director wanted.
- Same thing was done in The Abyss.
Gerry Anderson shows beginning with Stingray used dry-for-wet underwater scenes, in which model submarines or marionette divers would be filmed through a narrow water tank containing bubble machines and fish.
For Your Eyes Only: Many of the underwater scenes (including the close-ups between James Bond and Melina) were filmed on a dry sound stage and had lighting effects, slow-motion photography, wind, and bubbles added in post-production. Actress Carole Bouquet (Melina) had a preexisting health condition that prevented her from filming underwater.
The underwater scene in The Spirit was filmed dry.