It's interesting how much this doesn't, on an emotional level, actually feel like an enormous negative update about their situation. It's obviously correct for Merrin to be calm and matter-of-fact about it, but it's also not even hard.
(It's probably relevant that one of the possibilities here - specifically, Aspexia Rugatonn showing up with a Gate - seemed vastly worse for Merrin's future prospects than just eventually dying of micronutrient deficiencies on a weird exoplanet, and that seems - well, she can't rule anything out, she does not understand the causes of any of the things happening enough to be confident, but it seems significantly less likely. ...It might also be relevant that being upset and stressed sounds unpleasant? And Merrin, as much as anyone, prefers to avoid unpleasant hurty thoughts, and so if her brain is going to helpfully do some dissociation for her, she's not going to argue with it on that matter, at least not until the straightforward urgent work is done.)
There are...a lot of plan updates to be made, and some of them are going to take careful thought and a fully awake and reasoning Laeirthe, but Merrin's immediate thoughts are concrete and logistical. They're...actually fine on the scale of the next planetary month, she thinks? The shelter is good, summer won't kill them. She already knows what she can eat from local biomass and where and how to find it. Water is the only survival constraint requiring any really time-sensitive actions.
It's still daytime, and it's still going to be daytime for a while longer - 53 days after Merrin's arrival on-planet, the days are now almost 40 hours long, and the outside temperature was irrelevant until just now but it's going to be heatstroke-risk temperatures from now until well after dark. On future days they can simply wait to do still-refill trips until the cool part of the night, but the problem with that right now is that then they wouldn't get still-produced water until the next day is heating up, which, because this planet is awful, would mean waiting like 48 hours.
(Also, if Merrin were in Estha's position right now, she would probably kind of want some space? So perhaps it's convenient that Merrin needs to do an urgent water-carrying mission, not that she's going to argue if he offers to accompany her but she's the one with the power armor.)
"Okay," she says. "I can make enough freshwater with my still, but I need to actually fill it with seawater and I should do that really soon, before my Endure Elements wears off and while it's still hot enough to make some freshwater before sunset. How long ago did you last top up the water tank, it'll disappear after 24 hours, right? ...I should eat as much of the Created food as we can while it's good, since I don't think we can actually preserve it with magic, though I already have ice from last night in the icebox so we could try that. I still have the Share Language for a while, so I should memorize and I can write down as much vocabulary as I have time for, though I think I'm going to want to mainly speak the one you were Sharing before, I had time to actually learn the grammar structures for it. I'll need to plan a harvest mission on my own, but I have some preserved stores and some live snails to process, that's not urgent."
Pause.
"...Your magic items still work, right, and you don't actually need to eat? Do you need to drink water?" It feels pretty silly, now, that apparently she never registered the answer to that question, but it was just not at all relevant before, when they had Unlimited Magic Water, and now they don't.