The lead
One of the sounding leads from the Mary Rose.

When the lead hit the seabed, bits of stone, mud or small shells would stick to the tallow. This would help the pilot tell where he was, because the seabed varies from place to place.

The sailor whose job it was to use the lead stood on the bow of the ship and swung the lead out in front of it.

When the line was vertical he would measure the depth of water. The line had bits of cloth, rope and leather fixed to it to mark different depths. This let the sailor tell the depth by sight or at night by touch.

The depth was measured in fathoms, a fathom is 6 feet, or just under 2 metres.