Open water safety turns on where somebody holds the thermometer

Race cancellation limits are stated to a tenth of a degree, then measured by a person in a boat with a schedule to keep.

The rule is precise. Below a stated temperature the race does not happen; above another, it does not happen either; between them, wetsuits may be mandatory or forbidden depending on where the reading falls. Everything about the regulation projects certainty, and it is all resting on a probe held over the side of a rib at dawn.

Where the probe goes matters enormously. A lake in summer stratifies, so the surface can be several degrees warmer than the water a metre down where a swimmer's body actually sits. Sheltered water near the pontoon behaves differently from the exposed section at the far turn. Rules typically require readings at multiple points and a stated depth, and the ones that do not are effectively permitting the organiser to choose their answer.

Then there is the timing. Measurements taken at the referee's convenience, an hour or more before a start, describe a body of water that is warming, and the decision that gets made is about the water as it was rather than as it will be. That cuts both ways and organisers only ever seem to notice the direction that keeps the race on.

I do not think anyone is faking readings. I think a person holding a thermometer, with athletes who have flown in, a course laid, marshals in position and a timing contract, is under a pressure that no rulebook acknowledges, and the honest way to handle that pressure is to take the decision away from them. Independent measurement, published readings, a fixed protocol for depth and location, and a rule that requires the decision be made on the coldest of the recorded points rather than the average.

Triathlon has moved reasonably far in this direction and open water swimming has too, largely after deaths that forced the issue. That is the pattern with water temperature rules generally: they get written after somebody drowns, they get precise after somebody argues, and they stay unevenly enforced at the level below the international events, where the water is the same and the thermometer belongs to the person who sold the entries.

Precision in a rule is worthless without independence in the measurement. All that decimal place does is make the wrong reading look authoritative.