That implosion would have a localised short-term effect on water currents as water moves to fill the void, which may affect wildlife in the very immediate area, at the point of implosion (e.g. fish swimming directly next to the sphere may be sucked in and knocked into or trapped by the collapsing concrete).
I can't envisage a failure mode which would cause an explosion or cascade to other spheres in the locality.
In this example, the energy source is the pressure of the water column, which is equipotent with the depth, and applying equally to all external faces of the sphere. It'll be dramatic for the sphere, but fairly inconsequential for anything not immediately adjacent.