One huge advantage of ocean data centers is that you can do geothermal down there more cheaply and easily, and it's a very consistent source of power, so you don't even need hydro. Additionally, deep ocean water is consistently very near freezing, you could get below freezing out of the water but then you lose conduction efficiency.

The replacability is nice, but even in terrestrial data centers there are situations where something fails and it never gets replaced, just routed around, until the pod gets ripped out and replaced in its entirety.