Lead line

The simplest form of an echo sounder is a lead line, 60 feet long, attached to a 1 to 1,5 kilogram lead weight.
In the base of the weight is a cavity used for taking sea bed samples, which are useful when piloting or anchoring. The line has a series of marks along its length.
When dropped into the seabed, the marks are used to measure the depth of water.
The Lead line can be used as a backup when there are concerns about the reliability of the echo sounder.