People outside the US have a significant lack of Internet functionality when there are problems with submarine cables, but it hardly ever fails. Apart from when certain countries sabotage things.

If you only need 100M deep water then you don't need to go far offshore, there are places where it's only about 1KM out. 12M of water is used for storing spent nuclear fuel assemblies and less than half of that is needed for protection so 5M should be adequate for stopping cosmic rays and that depth is common in harbours for recreational boating, such harbours shouldn't have issues of cable breakage.

But the real issue for comparing submarine and orbital computers is cooling. Space is cold but doesn't allow easy transmission of heat.