Distances under water

How do I estimate them?

The refraction of light passing from water through your goggles into the air causes an optical reduction of distances as you tend to see objects closer to you than they really are by a factor of 3:4. In other words, an object that seems to be 3 m away is actually about 4 m away from you.

The same applies to your camera or camcorder.

As the cameras distance measuring system is normally also based on an optical system this phenomena has no effect on general snap shots. It does however affect close up shots (and statistically these are much more common under water than on land) where you might be tempted to use an arm length or measuring stick to ensure that the distance for the macro lens are being adhered to. In this case please remember: If the lens has been designed by the manufacturer for the camera to be 40 cm from the subject, multiply by 3/4 and then ensure that the object is 30 cm in front of the lens for that under water shot.