[mogoman]

Generally you lie in the water in a wetsuit for buoyancy. The spotter holds on to you to make sure you don't float out to sea (in our case). They also check on you by prompting you to give an "okay" indication (by lifting a finger every 30 seconds or so).

One danger of being under water is that you can pass out even after you've taken a breath of fresh air, so the spotter also is there to make sure you are good in the few seconds after you come up.