SC doesn't care about mermaid hydration motive.
I haven't had the opportunity to examine this yet. What does it do?
It's essentially the hygiene motive. Merfolk must regularly be in water to fill it up, when it's fully depleted they die.
I've also noticed that baths seem to fill it up faster than showers. Also standing in the rain or swimming does it, too. It's really more of an annoyance than a feature as it decays too fast. The only way it wouldn't be a pain is if we could build houses underwater...or at least let them sleep in water...but I digress...
Oh, and the Dirt Defiant trait supposedly doesn't affect hydration, but mermaids can still take it...I haven't personally tested this, but I saw complaints elsewhere about it...I certainly wouldn't cry if you added it to the list of motiveperkrebalance changes.
Mermaids apparently still have a hygiene motive, it's just hidden and (mostly) static.
Mermaid's also have a slightly different hunger system. Normal foods don't fill them up as much (although there's mods for that) and seafood and kelp is supposed to work normal for them. Not really sure if SC needs to be adjusted for that or not.