Mudlarks

Someone who scavenges in the mud for salvageable material.

"Mudlarks, so called from their being accustomed to prowl about, at low water, under the quarter of West India ships under pretence of grubbing in the mud for old ropes, iron, etc., but whose chief object was to receive and conceal small bags of sugar, coffee, etc. which they conveyed to such houses as they were directed, and for which services they generally received a share of the booty."

Patrick Colquhoun, A Treatise on the Police of the Metropolis (1796).