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.