The robbery happened in Bonhill, West Dunbartonshire, on Friday night as the man was night fishing in the River Leven.
The 48-year-old victim was fishing off the river pathway when he was approached by two men on bikes.
One of them pushed him into the water and the other made off with his fishing bag containing equipment and personal items - including a Rolex watch.
The two suspects are described as white, in their late 20s and wearing dark clothing. Both had baseball caps on, one of which was dark in colour with the other a lighter shade, possibly in a Burberry style.
Detective Constable David McKee of Clydebank CID, said: “The man had been doing a spot of night fishing in the area, something he does regularly there.
“The two suspects came up to him from the Alexandria side of the path and started chatting, but as he stood up to carry on fishing, one of the men pushed him into the water before the two of them made off with his belongings.
“The Rolex watch is an old style, one he has had for over 20 years and wears all the time. It is of great sentimental value to him and he is devastated by its loss.”
Police are keen to speak to anyone who may have seen the two men.
Anyone with information is asked to contact Police Scotland at Clydebank CID via 101.