One of these tribes is the "high-powered Serpentine splashers," who are politicians, media stars, and other influential people who swim in the early hours of the morning in the exclusive Serpentine Swimming Club.

Another tribe is the "Parliament Hill Concrete Stands," who prefer open-air pools, specifically the large Parliament Hill Lido.

These swimmers take their swimming seriously and enjoy the camaraderie and competition that comes with it.

There are several different tribes of people who swim in the UK, each with their own unique preferences and behaviors.

The first tribe is made up of the "Hampstead Bobble Hat Brigade," who swim in the ladies' pond on Hampstead Heath and wear bobble hats in winter.

The second tribe is made up of the "Thames-Traversing Nutters," who swim in the busiest river in the country, the Thames.

There are two sub-tribes of these nutters, the semi-legal swimmers and those who take bigger risks.

The semi-legal swimmers are those who take safety precautions, while the risk takers, like Matthew Parris, swim across the Thames from his Limehouse flat in the dead of night.

The article mentions a person who doesn't worry about swallowing water from the River Thames and finds it surprisingly salty.

It also mentions that the Thames Clipper is approaching fast and advises to look out for anonymous swimmers and Matthew Parris.