The Garmin Swim is a very neat little watch, runs on a standard coin-cell battery which lasts about a year, downloads to your computer via an ANT+ stick. Good day-to-day watch too. Been out several years now, so not as up to date as some offerings, no Bluetooth for example, but I’ve had great experience with mine.
The alternative is the Garmin Vivoactive, which does indoor swimming along with GPS for running/biking etc. A lot more features and probably a lot more for your money, but you’d need to charge it frequently which is about the only down-side I can think of.
Cheers, Rich