A lead line (with "lead" pronounced like the metal not like the first dog in an Iditarod team) is a line of rope (approximately 25 fathoms in length) attached to a tapered cylinder of lead of up to 28 pounds known as a sounding lead. The two in combination are used to determine with some accuracy the depth of water beneath a ship. The line was cast by hand into the ocean, sea, lake or river usually from the bow and as far ahead of the ship as possible. A deep sea lead would have a much longer line (100 fathoms) and can weigh up to twice as much as a sounding lead.
The lead was heaved out and allowed to hit bottom. The leadsman would then call out soundings for the master or pilot so that seaway could be determined and a course set. The actually depth was determined by a series of marks on the lead line which made it easy for an experienced seaman to know exactly how deep his line had dropped, even in darkness or bad weather. The points on the line, known as marks and deeps, were standardized in the 16th century and a little later the interesting doohickeys attached to the line became fairly common as well. The marks are usually, but not all together, odd fathoms while the deeps - which are unmarked - are usually even. But again, not always.
Mark 2 was two strips of leather attached to the line. Mark 3, three strips of leather. Then it's deep 4. Mark 5, a strip of white duck or bunting. Deep 6. Mark 7, red bunting. Deep 8 and deep 9. Mark 10, a piece of leather with a hole in it. Deep 11 and 12. Mark 13, a strip of blue surge. Deep 14. Mark 15, white duck. Deep 16. Mark 17, a strip of red bunting. Deep 18 and 19. Mark 20, two knots tied in the line. After that, it's all deeps.
It's easy to see why an experienced leadsman or, in smaller ships, a leadsman/pilot, would be so vital to the safety of even a very shallow draft vessel. Shoals, coral, rocks, sandbars; all could destroy a ship's hull and a lead read wrong could mean death or at least marooning.
Finally, for those of you who are thinking about it, there is a connection here to a rather well known American literary figure.
Samuel Clemens, who worked aboard ship on the greatest river in the world, took his pen name from the Mississippi rivermen's slang for two strips of leather attached to the lead line: Mark Twain.
Mind your lead, your marks and your deeps, Brethren. Tomorrow it's Friday Booty.