Alvin Clayton, 29, of Greenville went missing Sunday evening while swimming with a group in a small cove in the Skeena River at the east end of Terrace.
Police say he was last seen at around 7:20 p.m. PT diving into the water but failed to resurface.
A rescue boat arrived on scene within 30 minutes and continued to search until dark, but found no sign of Clayton.
Searchers were joined by eight other boats from the Kitsumkalum and Kitselas First Nations on Monday.
Meanwhile, the search will resume this morning for a Prince George man who is missing in Carp Lake, south of Mackenzie.
Police say the 59-year-old man, whose name has not been released, was not wearing a life jacket and is feared drowned.
The RCMP Underwater Recovery Team will be in the area on Tuesday to resume the search.