[iambecomelife]

You can set merchant ships' cargoes to Freight, Ammo, and Fuel in the mission editor, and I doubt that any mod maker would set the Hospital Ship to carry anything other than "Freight" (a catch-all term for non-volatile cargoes). The explosion was probably either the ship's own fuel supply or a boiler explosion. Boiler explosions were sometimes violent enough to break a ship in half, and a submarine crew might mistakenly think it was ammunition exploding. Sometimes when I hit a ship carrying Freight the explosion is just as violent as the one for a vessel carrying Ammo or Fuel.