An activity monitor that is worn by a user for monitoring water-based activities. The activity monitor includes at least one motion sensor, a water sensor, a memory, 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 processor can be triggered to operate to calculate the activity data and to save it to memory by an output from the water sensor to indicate that the monitor is immersed in water.