On our Apple Watch app, when you swim a workout loaded to your watch from your phone (we call this a "guided" workout), the app does it's best to calculate split times for each rep.
It should be noted that splits cannot be calculated for kicking, non-competitive strokes, and drills, usually because your watch/wrist isn't moving.
Splits start counting for a rep when you push off the wall, but only once the rep has started. So, to give the watch the best chance at accurately recording your splits, push off the wall only after you feel the haptic on the watch. That is to say, don't push off early (before the rep timer starts).