Floating poo in the sea; Poo in the sea
Johnny and his mates were stuck out on the boat drinking when one of them needed to take a dump. He had to do it in a bucket because they didn't have a working toilet. After jumping off the boat in the morning and wading back through to the shore (hungover) he is taking the bucket back up to the house to somehow dispose of the poo. He sets the bucket down and a few kids come running up to ask him what hes caught. Johnny grins at them and says 'Moon fish!' and laughs as they run over to take a look.
Moon fish is the name given to floating poo in the sea or river etc
Saw plenty of moon fish the other day when out fishing