I bought a Garmin watch to track my swimming. (Instinct2 - it does pool, open water & e-bike which is why I upgraded from the 735xt)
It's not actually that great at counting and the user error is shocking! I have lost count of the number of times I finish a set to find I'm still paused. They're also rubbish if you're doing drills - kick pulls are non existent and broken arrow/catchup can give false positives.
(If I'm counting I don't count numbers, I count letters. So the first 2 lengths are apple, then banana, cauliflower, daikon... Much less likely to lose track)
I'm averaging under 1.45 / 100m for the main part of the session, down from about 1.55/2.00 in December.
My swimming lessons have got me from 3:00 to 2:45.