An under water explosion creates a bubble which quickly collapses on itself. The action takes less that 1/100 of a second for bubbles less than a foot across. This creates a point of extremely focused energy at the middle point where the bubble collapses. In theory, this point focuses enough energy to trigger nuclear fusion.
Could the same effect be achieved by suspending a metal sphere under water which contains a near complete vacuum. If the sphere were opened uniformly along all of its surface area, the water rushing should create the same effect as the explosion, focusing all of the energy on a point at the center of the sphere.
IF not, then what are the limiting factors (surface tension maybe?) In my mind, It seems the major limiting factor is the shrinking surface area of the bubble as it collapses. as the surface area shrinks, the water molecules along the surface of the bubble would resist the change of surface area (some would need move away from the surface, in the oppisit direction of the collapse), causing the collapse rate to slow.