Why boats cannot stay on the pontoon moorings
This picture shows the pontoon moorings in the tidal River Witham at Boston, called The Haven, where boats moor before entering the lock at the entrance to the Black Sluice Navigation (left foreground). But it shows what the conditions are at low-water, when the mud-banks are plain to see. Of course, boats will not be (cannot be) sent down from the Grand Sluice Lock at this time of the tide but it adds to crew information to see the image.
In the background, upstream, Boston's fishing fleet is on its moorings. On the right, the frontage of Boston docks where sea-going coasters are often moored for cargo handling.