This is a term for a torture methodology. It is very popular, as it doesn't require a lot of paraphernalia.

Take a container of water. (This assumes you already have a victim.) This container could be a bucket, a washbasin, or a toilet, anything that can hold water and allow you to submerge the victim's head. (The water quality is up to the torturer.)

Step 1: Dip the victim into the water until they are about to lose consciousness. Step 2: Remove the victim's head from the water and allow them to recover. Step 3: Ask your question (or taunt the victim, you're the sicko doing the torturing, it's up to you.)

If the answer isn't satisfactory (or if you are a sadistic bastard), repeat step one as desired.

This procedure was used on Sean Penn's character in the move The Falcon and the Snowman.

The fact that this is called a "wet" submarine is to distinguish this method from the one called the "dry" submarine, in which a plastic bag is used as the means of asphyxiation.