mud-lark
A scavenger who wades the Thames mud at low tide, picking up coins, coal, bottles, and scrap.
“The mud-larks were out before dawn, clothes tucked to the knee, sifting the riverbank for what the tide had left.”
Note: Hotten's 1864 Slang Dictionary: mud-larks ‘grovel through the mud on the banks of the Thames, when the tide is low, for silver or pewter spoons, old bottles, pieces of iron, coal’ — a classic Victorian riverside figure.