Why is my Vivoactive showing incorrect exercise data?

If you are using an indoor activity the watch can not use the gps function. It instead relies on the accelerometer in the watch. This means that it counts every step and multiplies that by a given stride length for someone of your body type/age etc. If you have a longer or shorter stride this can cause inaccuracies. It is then recommended to set a custom stride length. This can be done on the app by:

Open the menu and go to the Garmin Devices page.

Tap on the vivoactive

Tap on user settings

scroll down and tap on stride length

It is recommended that you go to a location with a precicely measure distance like a 400m track and record and activity. Go around the track multiple times counting the number so you know the distance and look at the final step count to see how many steps it took. Then enter the distance and the steps in to the app. This will calculate a very accurate stride length for you.

Swimming is a different beast. It again is not using the GPS as water blocks the signal and most pools are short enough to fall within the margin of error. Instead the watch looks for arm strokes. It needs to see consistent strokes as you go down the pool and then a pause as you push off the wall. That pause is important as it tells the watch the length is finished and to start a new length. You can start and stop intervals with the left button. This is good to do if you are planning on changing between different stroke types.