The "Water Otter" feature at The Owl pub in Loughton, Essex (a McMullen's pub in Epping Forest) had been around for decades. It consisted of a water tank in the garden with a chain hanging into the water and a sign that says, "Please Don't Tease the Water Otter". When unsuspecting people pulled the chain, a kettle is raised, which is a long-standing practical joke that has amused children and adults for years.
You might be surprised to learn that this joke has been around for at least 150 years. First of all 'water otter' is an old pun-derived idiom used to describe kettles and other devices that heat water - they make the "water h'otter" . The idiom was widely used in England and was even adopted in brand names for water heaters.
There are a few accounts in the papers of these japes or hoaxes, usually not going down too well..
Dartmouth & South Hams chronicle - 3rd February 1882
Describes a long-winded jape where a beggar promises to secure a 'water otter' for a dog hunt and turns up the next day with it in a bag. The bag is revealed to contain a black kettle.
Banbury Advertiser - Thursday 24th April 1862
A similar jape as to the above.
Oxford Times - Saturday 23 January 1869
Another hunter with dogs is baited with this joke in Witney, Oxfordshire.