Grey Wagtail Nest

I have watched several pairs of Grey wagtails near a lock on the Calder & Hebble canal at Elland over the years and could never figure out where they nested. All was finally revealed this morning as I watched two adults periodically return to a nest under a large fern growing out of the lock with beaks full of food. I was astonished to find a nest within 6 feet of a frequently used lock gate. I wonder how many boaters pass this nest in a season and have no idea that a bird is nesting literally under their noses ! Does anyone take such records of nests locally ? I obviously don't want to put the exact location on a public forum.