Swimming lenght not accurate at all

I guess this clock is not for running in the water.

Correct.

Do you have any ideas if I can do something about this?

When walking/running in the pool, try faking a swim stroke with your arms. Doesn't have to be a particular stroke, just maintain a rhytmic monotonous motion. Do not break the stroke until you reach the wall. After turning back at the wall, stop the stroke for a second or two to let the watch know that you completed a length.

How do I stop the stroke? The problem with water running is that the rhytm is quite hard to keep similar, there are grannies that are too slow etc :D. But I´ll try that more monotonous motion. 

How do I stop the stroke?

Just stop moving your arms for a couple of seconds.

You can click the "Lap" button and it goes to pause. When you start another "run", click again "Lap" and the watch starts again...

But, as said, that (watch) is not the correct tool for the job (I don't know if there is one at all).

Whenever he does that, the watch will "restart" (another length) and add 50m, as it "thinks" that he has reached the wall.
It's like when I do 100 medley (4x25) on a 50m pool, it will always count 200m, as I change stile in the middle of the pool...

Your advices worked and the distance is ok now. But... what about the calories?? It certainly takes more calories to water run an hour that 63kcal. Do you have any ideas what´s wrong with that counting?

Well, do not bother with that, every watch  will give you a wrong estimation in that situation, strong arm movement and int he water, where HR will not be accurate. Consider get a swim (chest) band .

Garmin has a swim band, specially for pools…triathlon bands are not good for swimming in pools. However, if you do not want to swim…

Is Garmin HRM swim- band good with this clock?