If you want to have water evaporated, you want it to not contain too much gunk otherwise your “evaporation pot” will get muddy real quick (not defending ai, just giving their rational)

Water has to be balanced before being pumped through the facility, or it will corrode or calcify the piping. DCs use municipal water so they don't have to cover the cost of balancing themselves.

Plastic plumbing is only useful a) up to a certain size since it's not strong enough to support the weight and pressure beyond, and b) terrible at moving heat across, so it can't be used in radiators and definitely not in on-die cooling blocks.

Metal pipes can be coated on the inside to prevent corrosion, but that coating also reduces the amount of heat that transfers out - basically a layer of insulation. This is a big issue that had to be overcome for liquid sodium cooling in reactors, but those don't need to have a super thin copper plenum on a processor chip, it's larger pipes moving a larger volume of liquid through a significantly higher temperature difference.

However, salt water in a secondary, or even tertiary, loop could be used. It would be a similar concept to reactors, where the primary loop is 100% closed and uses a coolant designed for the task, and that then transfers heat through a reverse-flow chiller that's fed on the other side with salt water. The coolant pipes can be bare copper/aluminum/alloy for better heat transfer and the salt water side can be coated, and the surface area or flow volume just increased to compensate.

The downside though that can't be avoided is that your data center now has to be very close to an ocean, and the salt in the air could also cause problems.