>>674441. (LB)
NavAnon here.
Depends on the torp used. Many, but not all, sub-launched torps are designed to explode underneath the hull of a surface ship and create a big pocket of gas. The ship is designed to float using the surface tension of the water (aka "displacement"). Take away the water (with the gas bubble) and the ship can't support itself. Results in a cracked ship. We call it "breaking her back."
Remember, MANY but not all. Some have smaller warheads intended to punch a hole in the ship by contact. Most surface launched (called OTS or Over The Side) torps, as well as air-dropped torps, are the small warhead variety. It's a function of weight and size. Sub torps are bigger than most people realize.
Was it a torp? Limpet mine? Hard to say without more info. I suspect it was a limpet. But that's the whole point. Plausible deniability. The Iranians aren't stupid.