In 1774 a group of London doctors, concerned at the number of people who were mistakenly being given up for dead, wanted to promote new techniques of resuscitation. They decided to concentrate on drownings and formed The Institution for Affording Immediate Relief to Persons Apparently Dead from Drowning, on 18 April 1774 at the Chapter Coffee House, St Pauls Churchyard. It quickly became The Humane Society and in 1787 with George III’s patronage it became the Royal Humane Society.
The Society introduced what we might nowadays call life-guards at sites popular with bathers or ice-skaters (who mostly could not swim). Once the guard spotted a drowning person he would go out in a boat, fish the drowner out the water and use the doctors’ techniques to restore life. The techniques involved hot water, baths and beds so a building was required and a number of these were established in the Westminster area near popular water sites.
At the Serpentine an old farmhouse was used at first, on land given by the King. In 1835 this was replaced, on the same site, with a properly equipped Receiving House, designed by J. B. Bunning (who also designed the Copenhagen Park clock tower). This was damaged by a bomb in WW2 and demolished in 1954.
All the information above comes from the picture source, the Royal Humane Society and Pure and Constant which also has a drawing and a plan of the building. That website credits “Saved from a Watery Grave” by Diana Coke, published by the Royal Humane Society (2000).
The Receiving House is the building to the left in the picture.