Zoom in.

Take a picture of something out at sea. In this picture, you see mountains. Use a high resolution camera. When you upload it, look at the base. Then as you zoom in, look at it. You can also use a zoom lens at high resolution but make sure you are on optical setting. Now, after you downloaded it to your camera, zoom in. In all cases you can see right to the base line.

The above picture is 1.85MB, so click on the picture and get a closer look. You’ll be able to see down to the water line.

If you know the distance of how far that object is away, that would be better.