FWIW, a related bit of information is that
charts which show depths in fathoms will express them as, for example, 6-1 (with a smaller figure 1 than the figure 6) which is interpreted as fathoms plus feet, or a depth of 37 feet.
And, of course you will find
charts showing depths in meters which you can 'roughly' convert by multiplying by 3.
Most depth guages allow you to select the displayed units so you can synchronize the display with the chart but I find that a bit confusing so usually do the math in the head. With approaching age I may have to change the display :-)
Terry