IP68 only specifies water resistance below 1 metre, but exactly how far below 1m depends on manufacturer. Walking into the pool and then realizing, or having it in your pocket when you jump in, probably won’t cause an issue, but if your friend is holding their phone when you push them in, when the phone hits the water it’ll likely have water pressures above the equivalent of 1m and may potentially cause damage. Not saying it will for sure, and in most cases it’ll probably be fine, but it’s just worth noting. 1m of water pressure really isn’t very much at all. I know for watches, 30m is the bare minimum in most cases, because anything less than that isn’t even rain proof for heavier rains. A shower is around 50m of pressure I believe, though it’s been a while since I researched it. I’ll try to find a chart later today if I remember.

Edit: below is a generic guide for water resistance for watches. Obviously a watch is going to be more likely to make contact with water than a phone, and if your phone’s in your pocket, the fabric will obviously buffer a lot of that water pressure, but still.