It's hard to imagine anybody getting in there while it's underwater without nation-state level resources. We're talking about a sub with a huge moon pool that you park on top of it, or a specialized cover that fits over the hatch or something. You also have to deal with the pure nitrogen atmosphere inside of the data center. All in all it seems a lot harder than bribing the guards at a regular data center to look the other way while you mess around with some servers.

Attack sharks should easily solve the problem. In fact I heard that Steve Balmer himself was seen under the water, frolicking among his peers.

it wouldn't be too hard I don't think. It's a sealed tube so any attempts to get in could be detected with accelerometers or microphones.