Opposite to the Ramanathaswamy temple on the eastern sea shore is Agni theertham. Being one of the 64 sacred baths in Rameshwaram (theertham) every day thousands of devotees take bath in this holy sea. Many Hindu rituals such as prohithams, yangnya, tharpanam (ritual for the ancestors who are dead) are performed on the shores of Agni theertham.
It is believed that if any devotee baths in this sea then all their sins will wash away and if you mix the ashes of the dead person in this sea (Agni theertham) it will bring peace to the soul of the dead. If you plan for sacred bath in theerthas then you should first take bath in Agni theertham and followed by that bath in theerthas inside the temple.
It is also believed that if childless couples performing specific poojas (holy rituals) here and offer their prayers to the Ramanathaswamy (The main deity of this temple) they will be blessed with child. Although all the days in the year are consider as auspicious day for taking a sacred bath in this theertha but the amavasya (No moon day) and poornima (Full moon day) are considered as most auspicious days.
After Ram rescued Sita from the demon king Ravana Lord Rama Distrusted her purity. So Devi Sita got angry over Rama because of his suspicion and ordered Lakshman to build a funeral pyre and entered into the flames of fire. She prayed to Agni Devta that he shall protect her if she is pure. As Devi Sita was pure the fire did not harm her and instead made Lord Agni to appear there.
As by touching a chaste, pure and sinless women fire god Agni got a sin so he prayed to Lord Shiva and took a bath in the sea in Rameshwaram to vanquish his sin. The place he took the bath is called as Agni theertham.