Oh for goodness sake, I couldn't see it for looking!
It is of course a waste tank, the hut obviously containing a toilet. There is an electrical float switch in the tank indicating when it should be hauled away for emptying using the wire rope shown. When the tank is empty it requires the ballast weight and rope to keep it below the hut. It has to be allowed to surface when pulled away from the floating Khazi to permit emptying.
Which reminds me - we were discussing the problem of adapting a luxurious barge already converted for living, but which contained a waste tank below the new minimum Euro size and which could not readily be made bigger. An idea arose whereby a large dingy containing a tank could be towed behind the barge with a flexi waste pipe connected. Somehow it seemed appropriate to rename the barge AFFLUENT and name the tender EFFLUENT.