The information we find in the Ithihasas on the location of astras also seems to have the same logic. The astras are always got back from Varuna bhagavan! Varuna is the Lord of Oceans.The Shiva dhanus that Rama broke was got by King Janaka from a previous king Devaratha in a Maha yajna. Devaratha did not hand it over to him. It was in the custody of Varuna and Varuna handed it over to King Janaka in the yajna. Similarly there are references to Varuna in other places for either depositing the astras with Varuna or retrieving the astras from Varuna (Gandeeva of Arjuna and astras of Rama given by Janaka were kept in the custody of Varuna). If the astras were supposed to release some heat, then it is logical that they were deposited with Varuna.
Astras were different from sastras in that the astras were shot with some mantras or some innate power of the person who shoots it. The shooter learns it by some process of meditation or Thought flow. The Brahmastra and Pasupadastra were some well known astras which were shot with mantras by valiant persons who learnt them through some training. These astras were not always at the custody of the persons who shot them. After using them they were handed over to Varuna. Such kind of descriptions show that some powerful weapons that can be activated by meditation and mantra exuded tremendous heat that they can be safeguarded only in water! Perhaps the oceans and other water bodies housed them in times of non-use.