I want to understand the mission your envision where swimmers attached a large limpet mine to a ship underway.
Do you know that the Cheonan was steaming at the time, and at what speed? I don’t. She may have barely had way on.
The mine could have been attached when the ship was still in harbor. There may be an anti-submarine net at that harbor, but swimmers can easily go through it. Without the net the mini-sub could approach even closer; there were many attacks within a harbor in World War II, and that was when every side was most alert.
The reason for this scenario is simple - why would anyone need a dozen commandos to launch a torpedo? That, of course, matters only if the information leaked from NK is reliable; but it may be a carefully engineered disinformation, the major export product of NK :-)