Knot forms a marine hazard (4)
I believe the answer is:
reef
I believe this is a double definition.
'knot' is the first definition.
(reef knot is a kind of knot)
'a marine hazard' is the second definition.
(I've seen this in another clue)
'forms' is the link.
(Other definitions for reef that I've seen before include "portion of sail" , "Coral outcrop" , "The Great Barrier ..., Australian natural wonder" , "Reduce a sail's surface - shipping hazard" , "Ridge of rock or sand just below sea surface" .)