On Watch! is a watch-keeping aid and safety device used to prompt on-passage sailors and power-boaters to check their visual horizon at a user-selectable time interval. A series of escalating visual and audio alerts reminds the watch-person to do their horizon check without disturbing off-watch crew members. The watch-timer is reset when an alert sounds or whenever the watch keeper completes a horizon check. If the timer is not reset at the expiration of the alert period a pulsing alarm sounds waking the watch keeper and/or notifying the off-watch crew that something is wrong.