It's easy to overlook the effects of transpiration, which is a result of photosynthesis, which is impacted by biodiversity loss. It short circuits water's path back to the ocean and puts it back into the air above land where it can precipitate into lakes a second time.

Without it, snowmelt-fed rivers dry out earlier.

As more ice melts there might be more liquid water available, but the ecosystems that would process that water into vapor are struggling. So you end up with a situation where the only source of water vapor in the air is the oceans.

As temperatures increase that'll again be a net increase in ocean-driven evaporation, but it'll be less evenly distributed than evaporation+transpiration was.

So the concern is that the only available water will be too salty to drink, or too destructive to capture.