IRIS Dena was about 1500 tons displacement, similar in size to pre-Fletcher-class USN destroyers used in WW2 (except the four-pipers, which were a bit over 1200 tons).
The Mark 48 torpedo is designed to explode under the target's keel, breaking the target's back. The warhead is something like 650 pounds, fairly powerful. If one finds a video of the torpedo hitting the Dena and watch it closely, the hit was astern of midships, and the ship is blown into three large pieces (ignoring small debris). IIRC, the Dena was around 25 or 30 miles from Sri Lanka, meaning a rescue vessel could take around an hour to get to the wreck, if that quickly. By that time the large sections of the Dena would have been long sunk. The dead probably mostly died in the blast, were trapped in the wreck, or died fairly quickly due to serious injuries.
The Iranian navy was designed for coastal defense or commerce raiding/piracy. Those Iranian fast boats that had been harassing shipping and USN ships are now permanently wet-docked in their harbor or off the coast. or are splinters spread about a 50 or 100 yard radius.