Measure of Water, a Depth of Six Feet
Puzzle 12 of Sailing Hard Pack
Question is: Measure of Water, a Depth of Six Feet
and the answer is: FATHOM
Word Meaning:
FATHOM — (chiefly nautical, historical, US) A man's armspan, generally reckoned to be six feet (about 1.8 metres). Later used to measure the depth of water, but now generally replaced by the metre outside American usage.
Definitions from Wiktionary.