In the recent past, depths were measured in fathoms, although since metrication this has mostly changed to metres. Most U.S. charts use fathoms or feet; the U.S.A. also uses a different chart datum. Sounding lines were widely used in navigation until the development of ultrasonic depth-measuring devices. The sounding line has been superseded by echo sounding. Ultrasonic depth sounders can be cheap, accurate, and provide a real-time graphical profile of the depth of the seabed. To work continuously and reliably, they do need electrical power and shelter from rain and spray, something that can be difficult to maintain on smaller and open boats.
It is easy to measure a length of line or rope as a rough number of fathoms by repeatedly stretching the rope between the two outstretched arms. Water depths have traditionally been measured this way using a weighted sounding line. The word fathom can be used as a verb to describe this process.
At sea, in order to avoid repeatedly hauling in and measuring the wet line by stretching it out with one's arms, it became traditional to tie marks at intervals along the line. These marks were made of leather, calico, serge and other materials, and so shaped and attached that it was possible to "read" them on sight by day or at night by the feel of each one. The marks were at every second or third fathom, in a traditional order: at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. The "leadsman" called out the depth as he read it off the line. If the depth was at a mark he would call "by the mark" followed by the number, if the depth was between two marks, he would call "by the deep" followed by the estimated number.
On the Mississippi river in the 1850s, the leadsmen also used old-fashioned words for some of the numbers; for example instead of "two" they would say "twain". Thus when there were only two fathoms left under the boat they would call "by the mark twain!". The American writer Mark Twain, a former river pilot, likely took his pen name from this cry.
Sometimes tallow was placed at the recess in the bottom of the plummet to pick up traces of any loose material (such as mud, sand, or shingle) from the seabed, providing information useful for pilotage and anchoring. If the plummet came up clean, it meant the bottom was rock. Nautical charts now provide information of the seabed materials at particular locations.
P. Kemp, ed., The Oxford Companion to Ships and the Sea (London: Oxford University Press, 1976).