This page of the guide to AC Odyssey shows Ainigmata Ostraka on Messara. There is only one called Till Death Do Us Part.
The location of this Ainigmata Ostraka: The first part of this Ainigmata Ostraka is in the Temple of Poseidon. A small water body can be found on the right side of the temple - the tablet is inside of it.
Puzzle solution: Head south and look for the tall tree. The item is on the blanket located right next to that tree.