I know that a fathom is a unit of measure used by sailors, but how long is a fathom?

A fathom is a unit of length used to measure the depth of water or the length of nautical rope or cable. A fathom is six feet, or 1.8288 meters.

In Treasure Island, by Robert Louis Stevenson, Dr. Livesey helps gather supplies as they flee from the mutineers:

Pork, powder, and biscuit was the cargo, with only a musket and a cutlass apiece for the squire and me and Redruth and the captain. The rest of the arms and powder we dropped overboard in two fathoms and a half of water, so that we could see the bright steel shining far below us in the sun, on the clean, sandy bottom.