A "fathom" is a unit of measure that is US 6ft or SI Metric 1.8m. The use of a fathom is nearly obsolete as any nautical chart or citation using it as a unit of length will convert it to SI metric prior to release to Navy or civilian sectors. Oddly enough people use "fathometers" as a depth finder gauge but the use of "fathoms" on such tools are usually found on more expensive models... but no one really uses it for "fathom" measurements and only US Hydrographers use it but again, convert it to SI Metric prior to release to Navy or public. Go figure.

Thus, a fathom is rather useless since it will be converted one way or another. I know so, I did nautical charting for a few years and had the honor of converting every piece of someone's hard work done at sea. Click the meter button next time you butt pirate!