This word can have other meanings but it mostly refers to the riverside equivalent of the beachcomber, who searches in the mud at low tide for anything of value.
These days they will probably be hobbyists armed with metal detectors, looking for items of historical interest (one recently turned up a rude medieval brooch in the Thames mud, probably a token from a nearby brothel). Their predecessors were of a quite different sort.
The word came into use near the end of the eighteenth century to refer to destitute Londoners who picked over the Thames foreshore. They sought out lumps of coal, bits of old iron and other detritus that fell from ships in the port of London — anything which they could sell for a few pence. They were most vividly evoked by Henry Mayhew, in London Labour and the London Poor:
They may be seen of all ages, from mere childhood to positive decrepitude, crawling among the barges at the various wharfs along the river; it cannot be said that they are clad in rags, for they are scarcely half covered by the tattered indescribable things that serve them for clothing; their bodies are grimed with the foul soil of the river, and their torn garments stiffened up like boards with dirt of every possible description.
More sinisterly, they were not above a little light thieving from moored vessels, and were also used as go-betweens to people on shore by sailors who had smuggled in expensive commodities such as sugar or tea.