Swim workouts

you only press the button at the end of the last distance. If you have a workout that has a repeat of 200yd x 4 sets w/ 20 sec rest between, the watch will vibrate when you are on the last lap of each 200yd set. When you hit the wall at the end of the last lap you press the lap button. The watch will then start your 20 sec rest countdown and then start your next set timer after the rest countdown is complete automatically.

The watch doesn't know whether you're in the water or not, try it at home. it'll record 0m for your laps but you can see what phase you're in on the watch and count the laps afterwards :)

Just to be clear, I think we need to define terms. When you say "doing a swim workout", you mean a pre-planned workout - i.e. you have created a workout on Garmin Connect and sent it to the watch and you are now doing that workout in the pool, right? In that case, if you have set up a rest time (e.g. 20 seconds) then the watch will count you in with beeping and display "3", "2", "1"... If you get a count in like that, you do not need to press Lap (as the watch assumes you start swimming when you are told to). If you do not get counted in (perhaps you have set up to rest until "Lap Button Press"), then you need to press Lap as you push off. (The rest timer is also a hint - if it is counting up, you aren't going to get a count in. If it is counting down, you are.)

You always need to press Lap when you get to the wall to start your rest.

If you are doing swim sets in the pool but not a pre-planned GC workout, then you will never get a count in and you always have to press Lap when you arrive at the wall to rest, and when you push off again. Same as the 910. One improvement is that the screen inverts (light text on dark background) when you are in rest mode.

Hopefully I haven't caused more confusion! The tip to give it a try on land is a good one.

just adding what happens with my Fenix3 (I'm assuming it's the same).

you just need to start the workout, and then press LAP when you are about to enter a REST. Then you just need to STOP when you finish the workout, everything else is automatic.

also, when you are on the final lap of any given interval, the watch will buzz.

for all swimmer

For @onmyway... and for all that didn't have 920
It's not so easy to understand how the swim workouts work, so I want to suggest to try during the warm up and the cool down some workouts that you created before in GC. So you can swim during your main program but at the same swim session you start understanding the right use of the workouts.
After understanding it's very easy to use and the garmin watch can help you to improve your swim sessions, without thinking "how many laps I did...." or "how many intervals...".

I'm using my 935 for triathlons and did previously have 910XT where I had to press every time I arrived after the distance to mark it "finished". When I then started I pressed the lap again in true 910 xt style to mark the start on the next distance I should swim.

When I was doing swim workouts today I did as I use to do, press on arrival and on start, but then I have loads of zero laps and my intervals all if a sudden became two instead of four. The whole training went bonkers. I did some ad hoc training to save some of the water time.

How do you swim with a workout and the 935? Ie do you press any buttons at all and if, when?

Swim workouts are not that useful but you only push the lap at the end of the interval, the watch should beep a few times before you need to take off on the next interval and start by its self..