Fish shit

The long excretion from one's penis after penetrating someone anally and before producing a urine stream.

Sitting with your bare arse over the edge of a boat, and firing the chocolate spud gun into the water.

when a fishy jerks off into a cup

The Brittish version of the sexual act better known as the shocker. Both the index and middle fingers are inserted into the vagina while the pinky on the same hand penetrates the anus. Two different and distinctive aromas are left on the fingers upon removal.

Those who watch Buzzfeed Unsolved a show hosted by Ryan Bergara and Shane Madej typically fall into one of two groups each named after one of the hosts. Those groups being Boogaras, the believers, or Shaniacs the non-believers. If someone fails to fall into one of the two groups or claims to be in both they are labeled as being a Shit Fish.

Merchant Navy or Trawlers
Any of several species of fish that congregate around the sewage discharge pipe of stationary ships or boats to feed off fecal matter that is discharged overboard.

When somone puts their arm down the piping of the toilet to find a shit. This is usually performed after somone else has just been to the toilet, obviously increasing the chance to find something.

There are many reasons people do shit fishing, but the three most commen reasons are:
Blocked toilet
Somone ate something that the person needed
Hunger