I'd hoped to swim today but work was too much work. On the walk home the Garmin Swim flashed up 'Low Battery'. Although this has been the first warning I don't want to lose data so swapped the battery this evening. I had spare CR2032 cells in stock as they are one of those sizes that fit a multitude of gadgets nowadays.
The swap was easy as per the instruction manual.
The estimated battery life is 1 year, my first use is recorded as 14th July 2015 so that's a pretty reasonable prediction. I've barely used the light which I'd assume is a substantial drain.
I hope the seal is OK, the O-ring is slender!