It is almost a year since the Garmin Swim launched so there are likely to be some watches out there with batteries that are starting to run low. The estimated battery life of the Garmin Swim is one year, depending on how the watch is used. When the battery starts to run low, there is a battery icon at the top of the display that will illuminate. As the battery continues to get lower, the watch will start popping-up "Low Batt" warnings.
The watch uses a CR2032 battery that you can replace yourself. However, if done incorrectly it is possible that the water resistance can be compromised. Instructions on how to replace the battery can be found in the Owner's Manual or watch this video. The critical step is to make sure the red gasket around the door gets properly seated before the door is reinstalled, but please watch the whole video... :-)