I don't for polar and if I did I'd never get any swimming done, it detects rest automatically and gives you overall time + actual swim time recorded. It calculates swolf and avg pace by moving time and pace recorded when in motion.

Strava doesn't quite get it when polar uploads and just says avg pace from the overall time, which isn't correct - you don't smash out a set then include the rest time in the pace.

Like running intervals - you take the pace/target pace from time actually spent doing the interval, not the recovery between efforts.

I wouldn't worry about recorded average pace starting out and doing workouts anyway, if you have a structured workout with drills and stuff the average time is useless anyway, per lap/length time is more important - use the wall clock if you really want to know.

Stick at it, build endurance and work on drills, try smaller structured workouts and get a kickboard/pullkick bare minimum then pick a goal.

You can try time trialling a set distance and using that as your comparison benchmark - a common test is doing 1k and working out the average 100m/y pace by ÷ 10.

I can run 3h no problem but after swimming two laps I'm already gasping for breath lol

I was exactly the same, half marathon no issues and trail running lunatic. Tried a 15m hotel pool and nearly died trying to get 10 lengths done.
When I tried the big boy 50m pool I did two laps and nearly went home and cried because I was so shit at swimming.

It's been just over a year now and in that exact same 50m pool today I punched out 5k in 1:29.

My best advice would be focus on the swim form, technique, breathing, drills and relaxing as much as you can in the water while ignoring the metrics - they'll work against you and self esteem in the water. Improvement isn't all about pace.