How many?

Fathoms

A fathom is a unit of length in the imperial and the U.S. customary systems equal to 72 inches (1,828.8000 mm), used especially for measuring the depth of water. The Atlantic Ocean at its deepest point is believed to be 4,580 fathoms deep.

Nautical Miles

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters.Historically, it was defined as the meridian arc length corresponding to one minute (1/60 of a degree) of latitude. Today the international nautical mile is defined as exactly 1,852 metres. The derived unit of speed is the knot, one nautical mile per hour. To sail from Southampton to New York would cover a distance of around 3,081 nautical miles.