I read that in order to change a pet to Hydrus, you have to get all the map pieces and take a quest from Marana. I just bought all the map pieces and put them together. However, she isn't on the Omen Islands? And there's no mention of what times Marana is available that I can find... Is it like, an annual thing? Do I have to wait forever to turn my pet Hydrus?

Merana appears at random times on the map now. You can join the "hydrus" forum group to get pinged when she appears.

https://subeta.net/forums.php/read/869600/Merana-ping-group-hydrus

Thank you very much!

where do you read that you need map pieces? Merana has escaped, she's no longer trapped in Omen.