Considering the lacking stamina, they didn't travel more than 50 meters before the young stingray crashed on the sand encrusted coral and refused to move.
Thankfully he didn't faint but the boy would be close to it if they swam just another 50 meters.
But before he crashed on the sand, a little crack in the coral managed to capture the corner of his eye.
So after Osira was under a bit of panic, they made a decision of taking refuge in that crack.
Unlike on most corals he had explored, this crack had the entrance right on the very top. So it means that the formation was either cracked from a bigger being or was created like that naturally.
Either way it didn't bother them as taking refuge in it became the prime idea, the architectural values didn't really matter.
They struggled to climb in it at first, mainly because of Timmy having so little stamina and of course because the crack was high on top of this strange circle shaped coral.