A mudlark is someone who scavenges in river mud for items of value, a term used especially to describe those who scavenged this way in London during the late 18th and 19th centuries.
Oxford English Dictionary. Third edition, March 2003; online version March 2011: According to the Oxford English Dictionary, first published use of the word was in 1785 as a slang term meaning 'a hog'. The dictionary speculates its origin may have been a humorous variation on 'skylark'. By 1796 the word was also being used to describe 'Men and boys ... who prowl about, and watch under the ships when the tide will permit.'