Coastguard and police officers are searching the promenade area in Cleethorpes after a report that a woman may have entered the water in the area.
An unknown man reported concern for a 45-year-old woman who he believed had intentionally gone into the sea.
However, the man left without providing his full details or the circumstances of the incident.
Officers would like to speak to the man to establish the full circumstance of his concern and help find the woman.
The man or anyone with information is urged to call Humberside Police on 101, quoting incident number 698 of October 30.