after a mooch right along the dock wall as far a i could go i spotted this and returned with good old kevsy21 and had a little look up on top over bramley moore docks
id already done the one in the east float docks but what made this different was that it wasnt properly secured and kept floating out and coming back in crashing against the dock wall
anyway heres what they do
The Caission is used to close Dock / Lock entrances when the gates are off and being repaired. It is floated over the lock / dock entrance and then flooded down onto the gates threshold where it seals the entrance to the waters.