Partially softened hard water is fine to use. Fully softened water isn’t recommended.
The UK’s Pool Water Treatment Advisory Group advises to maintain calcium hardness of between 75 and 150 ppm as CaCO3.
The problems of fully softening pool water:
If you soften below 75 ppm, the water will slowly dissolve the calcium from grout on a tiled pool and etch any calcium-based coping stones. Fully softening also makes it harder to maintain the pH balance of the pool water.
How to soften pool water to recommended levels:
For a typical 300 ppm pool in a hard water area, a block salt water softener can be used to soften the water.
This should be done over a number of days as the softener needs the time to make brine from the salt blocks. Half fill the swimming pool and then soften the remainder to give you a half and half solution. Keep the level of hardness below 150 ppm.
In an area of about 400 ppm, run the softener at 20 litres a minute for 30 minutes, then stop for at least an hour. Start again and repeat as required.