The rescue is the story, and knowing it changes the visit. After the first Aswan dam was completed in 1902 the temple spent most of each year submerged; tourists were rowed between the tops of the columns and looked down through green water at the capitals. When the High Dam was planned it would have gone under permanently. Between 1972 and 1980 a UNESCO campaign built a coffer dam, pumped the site dry, numbered and dismantled the whole complex, and reassembled it on Agilkia island, reshaping that island to match the original topography. The joins are visible if you look for them.
It is also the last surviving corner of ancient Egyptian religion. The cult of Isis continued here for centuries after the rest of Egypt had turned Christian, and the last dated hieroglyphic inscription anywhere — AD 394 — is carved on this site, along with the last known inscription in demotic some decades later. The temple was finally closed under Justinian in the sixth century.
And the building itself is unusually easy to like. It is Ptolemaic and Roman rather than pharaonic, which means it is smaller, more decorative and better preserved than the great New Kingdom temples, with a kiosk on the water's edge that is the most photographed structure in Aswan.