if you do it enough you know the difference between 12 low, 12 mid and 12 high or whatever.

Unless you are trying to move from 22.4 to 21.9 in the LCM 50 this is accurate enough (you get the point).

What's wrong with a watch?

Watch would not be accurate enough, If I am targeting 12.5 , the time taken between starting the watch & pushing off + finishing and stopping the watch , would be over 10% of the total time , I don't want to live with that level of uncertainty :)

I have a Garmin, so I can't speak on Apple Watches, but mine has a three second countdown before it starts. Since you are hitting the stop button every time, you will be consistent with that. As long as you stay consistent with how you measure it, you will be able to detect deviations for better or worse.

Good luck.

If you really want to know your times down to 0.1s, sounds like you need to get your own timing mat and clock. I'm fairly certain that coaches doing USPRT with a team aren't timing all of their kids on deck and letting them use the pace clock.

This is why normal people hate triathletes

Are you really busting out a bunch of sub 13 25's with that level of consistency?

Yup use the countdown and you can set a time alert which will vibrate at whatever u set it to.

Does your Y have a pace clock? Most do

Depended on by swimmers all over the world for decades, highly accurate, highly durable and comes with a life time warranty ('hey, can you pls put the clock on when you have a sec / when you have a sec can you align the 2 clocks'), great value for money as comes free with pool entry :)

Doing rounds of 16-20 x 25 on about 30 gives you insane insight into pace. You want to have a sense of your timing on more usrpt sets, but doesn't need to be super dialled in. Main thing is swimming at top speed!
but hey a mobile touchpad at your local y is a sure way to start a convo, probably a sure way to get ex-swimmers to heckle / rip you one too

I’ve incorporated Race Pace/USRPT sets into the team I coach for approaching a decade now. You don’t need anything more than a pace clock on the deck.

I hope this helps,

Tim

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Regarding pace clocks, analogue ones, at least:

As I think about it, the huge radius of the second hand on these things might even make them an especially accurate option for seeing parts of a second.

(BTW, I feel literally sick right now thinking of a pace clock. I can dread the next sendoff from 30 years away...)

What event(s) are you training for?