The 26-year-old, from Blighs Gardens in the city, has been missing for the past nine days.
Searches have been carried out along the border with Donegal after possible sightings of Martin who is described as 6ft 1 inches tall.
A family friend posted on Facebaook: “The family and friends of Martin Crumlish now believe he entered the Foyle.
“We are appealing for volunteers in the search for his remains. As you can appreciate this is a very hard time for all concerned.
“Searches will be coordinated from down along the river bank at the Coshowen end.
“The search will stretch from the Daisyfield to the new bridge.
“If you have a spare hour or two please come along and help bring Martin home.”
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