Sea water decomposes plastic faster than it corrodes metal. This is one of the issues with plastic pollution of the oceans they break down release nasty chemicals and end up as micro plastics.

Most plastics will decompose under a year in the oceans turning it into soup basically. http://news.nationalgeographic.com/news/2009/08/090820-plast...

Interesting - I didn't know that. But the article you cite says it only really happens in tropical waters above 30C (86F). Is there evidence plastic decomposes in sea water in cooler climates?

By definition if the water is used for cooling, it will be warmed up when flowing through the datacenter.

But you could use sea water just a secondary cooling system, with non sea water flowing through the datacentre and a heat exchange between the two.