Is anyone still seeing this issue ?
EDIT: Going to mark this as fixed. If anyone still sees it, please speak up.
I've had that happen too! My mermaid had been sleeping in the water for a long time and at this point, her hydration was full. Then the other sim in the household experienced a glitch with her actions queue (her phone filled it up, much like the Pets glitch before I think). It might not be related, but once it was solved I checked to see if the mermaid's queue was doing the same thing. It wasn't, but I noticed that she was steadily losing hydration. I thought maybe the sleeping interaction was glitched and I sent her to swim around, but it only increased the speed of decrease, AND she was swimming like a human sim.
What solved it was telling her to get out of the water. She changed into human form, then I sent her back to swim again and the problem was fixed, she was gaining hydration and swimming normally. I haven't experienced it again since. It might definitely be linked to how long they spend in the water.
I'm on a mid-2015 MacBook Pro and playing completely vanilla!
