How to setup Timed Interval for swim workouts

Just create a new pool swim workout, inside a 4x Repeat loop set the swim to 25m and the Rest to "Fixed repetition duration" of 30sec.

thank you!

When I'm swimming this workout, do I need to do anything between repeats? 

yes every time you finished swimming (in this example after 25m) you need to press the lap button to signal to the watch that you arrived at the other side of the pool.
During normal swimming, the watch is able to count the number of lengths that you did by detecting the push off from the wall. After the last length of a set, you do not push off again but you stay for the break which is why you have to tell the watch manually that you are done.

Ok, thanks. 

So just to make sure that I'm not missing anything: 

1. Does it mean that in the above example, let's say that I finish the lap in 20 seconds when pressing the lap button it will only count 10 seconds before alerting me to go for the next one right?
2. When I'm doing 4*100, I need to press the lap button only at every 100m right? 

Thanks in advance - this feature is one of the main reasons that I bought this watch, and the user manual is pretty unclear. 

Yes exactly! If you swim 4*100, you only press the lap button after every 100m so, assuming you swim in a 25m pool, every four length. During the last length the watch will also notify you by vibrating that you are on you last length so that you don't even have to count. It works super well for me and it rarely happens that the watch miscounts, mostly if I have to stop within a length because of other people. Just make sure to have a good push-off the wall followed by a nice gliding phase as this will help the watch to detect the beginning of a lane

Hi -Super helpful thread. 

Does the watch prompt/ auto detect start or do we have to manually press lap after each rest period (using fixed repetition duration). 

No need to press the lap button - as soon as the rest period ends it signals you to start the next interval