How is an open water race timed?

• Two transponders: "All competitors must start the race with a microchip transponder on each wrist."
• Losing one: the race judge or another official tells the chief referee, who has a replacement issued on the water.
• At the finish: "Any competitor who finishes the race without at least one transponder will be disqualified."
• Tenths: "Microchip transponder timing technology will be recorded officially in tenths of seconds."
• Precedence: results from the automatic equipment decide the winner, the placings and the times, ahead of the finish judges and timekeepers.
• If it fails: the judges' and timekeepers' records become official. A swimmer with no automatic place is placed by the chief referee from the video of the finish.
• Hand timing: three timekeepers, to a tenth of a second. Two matching watches give the time. If all three differ, the middle one counts.
• Splits: transponder systems must be able to give split times.
• Synchronised watches: a time check is made 15 minutes before the start.

The glossary covers this under open water time limit and transponders. A time for part of the race is a split.

There is no lane and no wall for each swimmer, so the clock has to be carried on the body. Two chips give a spare. Tenths, not hundredths, are as fine as a measurement taken on moving water over a course laid with buoys can honestly be.