Google's datacenter in Hamina, Finland uses salt water for one of the cooling loops. aiui it was not trivial to incorporate the salt water loop (corrosion was an issue) but they've made it work.
Google also has a 30 percent profit margin, even after accounting for boondoggles like that.
Building a system from the ground up to use saltwater is a whole different ballgame then trying to retrofit an existing legacy network though.
It would probably be cheaper to just convert to another heat source all together.
Water in a datacenter cooling loop is almost certainly not heated to the same temperatures as water in a (steam) heating loop. Corrosion is temperature dependent.
And on top of that, the Google datacenter in Hamina doesn't use ocean water with 3.5% salinity, but water from the Baltic sea, which in that part of the Gulf of Finland only has roughly 0.3% salinity – a tenth of ocean water.
Sounds like less of a Baltic sea thing, but more of being in a fjord or bay that's fed by fresh water keeping a lot of the salty sea water out.
You can get basalt-lined pipe, for chemical stability.
Even without reading TFA it's surely not the loop what is actually cooling the hardware.