An activity monitor that is worn by a user for monitoring water-based activities. The activity monitor includes at least one motion sensor, a memory, a battery to power the monitor and a processor. The processor is operable to run an algorithm to calculate activity data based on an output from the motion sensor and to save the activity data to the memory. The monitor has a sleep mode in which the monitor consumes little or no battery power, a standby mode in which power consumption is higher than the sleep mode but in which the processor is not operating to calculate activity data and an active mode in which the processor is operating to calculate activity data. The activity monitor is responsive to a first trigger to switch from sleep mode to standby mode and responsive to a second, different trigger to switch from standby mode to active mode.
