When navigating waterways with unknown depths a lead line was used to quickly give measurements in order to keep the vessel from running aground.
Printables:
Additional Links:
An explanation of how the Lead Line was used.
https://seahistory.org/sea-history-for-kids/leadline/
A quick tutorial for figuring out a depth at sea in the age before electonics.
https://www.youtube.com/watch?v=RDE4SlYdn5E
