First of all, a ship at sea is not always easy to communicate with, especially an older sub with balky systems. Additionally, any fire aboard a sub can be exceptionally dangerous, particularly in the battery compartment where there is plenty of acid to create toxic vapor. If submerged at the time, matters are even worse and can escalate rapidly. Inhalation of hazardous smoke and other released vapors may not show the most severe symptoms for a period of time, giving rescuers a false sense of well-being. And last, but not least, we probably wouldn't hear of a fatality before the family was informed (which wouldn't happen until verification of the identity of the sailor) - a day is not unreasonable.