Garmin 910xt

Yes that's it,could I pester you and ask how do you reset lap length again when the workout is under way

You shouldn't have to do this (unless you are changing to a different pool mid-workout-- I'm going to assume you are in one pool )

Before the workout you set the pool length, not the lap (interval) length. The watch will automatically count lengths in the pool. You don't change the pool length in the middle of the workout. You hit the lap button at the end of each interval and after the rest intervals.

So for example, I swim in a 25m pool. I set the pool length at 25m.

Last night, my warmup was

1 x 200m with :20 rest
2 x 100m with :15 rest
4 x 50m with :10 rest

I pressed start at the beginning of the workout, swam 8 lengths, and then hit the lap button. Hit the lap button again after 20 seconds for the rest.
Then I swam 4 lengths, hit the lap button, hit the lap button again after 15 sec rest, 4 lengths again, hit lap button, hit lap button again after 15 sec rest.
Then 2 lengths, hit lap button, etc.

The pool counts the lengths within each lap and records the time. The workout looked like this:

http://connect.garmin.com/modern/activity/635744542

The "intervals" correspond to when I hit the lap button. The lengths are automatically counted within each interval.

(If you're wondering why some of the rest intervals are very long, those are when I was doing drills and the watch thinks I'm standing still. I want the 920xt so I can fix this. )