St John’s and other pills are discussed on this page, where I said
The term is associated with south-western England and Wales but it is also found in south-eastern Ireland.
Reading Hugh Conway-Jones’s excellent Working Life on Severn & Canal: reminiscences of working boatmen (Alan Sutton, 1990), I noted this:
By the time you got to Avonmouth, the tide was more than half-ebbed out and the dock gates were closed. So the tug took you round into the Old Entrance (what we called the Old Way), and you tied up there alongside the pier. As the tide ebbed, the barges lay on the bottom and then you could get your head down for a nice sleep while the tug went off to Portishead to anchor in the pill there.
I’m not sure whether this Pill is the one the boatman had in mind: although the two places, Portishead and Pill, are often mentioned together, it seems a bit far away to be regarded as Portishead’s pill. Further information welcome — and sightings of any other pills from either side of the Severn.
Incidentally, the sort of traffic and conditions Conway-Jones wrote about seem to be similar to those on the Barrow and Suir estuaries (allowing for their smaller scale).