What are an umpire's safety duties, including capsizes and bad weather?

The Rules say the safety of rowers must be the prime concern of the Jury at all times (Rule 91).

• Capsize or swimmers: if a competitor falls in or a boat capsizes or sinks, the Umpire must satisfy themselves that the rescue service is in action. If it is not, they remain with the capsized crew until all members are safe and the rescue service is present.
• Squalls or sudden bad weather: the Umpire decides whether a race may be started, continue or be stopped. The By-Law states that the safety of the competitors is more important than any of the provisions of the Rules of Racing.
• Stopping a crew: the Umpire may raise the white flag, name the crew and call "Stop" to protect people and prevent damage to boats and equipment.
• Unsafe crews: any member of the Jury may prohibit a crew from going on the water if they consider it a danger to itself or other crews. If there is a dispute, the President of the Jury decides.
• Unsafe conditions generally: the President of the Jury may take any necessary decisions and pass changes to the Starter and the Umpires on the course.
• Zonal umpiring: if it is about to put crews' safety at risk, the President of the Jury may revert to dynamic umpiring (following the race) for the rest of that racing session.

Sources: Rules 63, 91, 98 and Appendix 13 By-Law to Rule 97.