Plus, at depth, storms and typhoons don't affect things all that much, it's rather calm. So, the main threat might be from saboteurs rather than natural disasters [beside the salty environment] because beside a coast guard at the surface [which if contraband coming in is any proxy, it's pretty porous], you don't have a "police presence". So they'd have to rely heavily on monitoring systems.
If you need to fix something while the datacentre is in the middle of the ocean, the cost and time will be a multiple vs if it just off the coast.
http://blogs.discovermagazine.com/d-brief/2014/08/15/sharks-...
Quick googling yields [1] datacenter land selling for more than $1 million per acre in SV and [2] Google's requirements for datacenter placement. The first four points listed are cheap electricity, carbon neutrality, lots of water, and large parcels of land; the 215 to 1200 acres mentioned in [2] would cost $240 million to $1.5 billion at the price quoted in [1]. Sealed containers anchored to the free sea floor, running on free wave energy and cooled with free sea water would be a very clever way to satisfy those requirements while staying close to the customers.
[1] http://www.datacenterknowledge.com/archives/2015/07/24/equin...
[2] http://www.datacenterknowledge.com/google-data-center-faq-pa...
Latency.
Suppose you have a bunch of people somewhere, say, the US, and a bunch of other people somewhere else, say, China, and there's an ocean in between. If they need to work collaboratively on something, placing a datacenter in one country or the other yields asymmetric latency; someone has a lot more.
If you can just plop a datacenter exactly at the midpoint, everyone wins. It needn't be the biggest datacenter ever, just one that can handle the latency-sensitive tasks.
Neat project.