Okay!!! It seems like they totally in fact failed to be on the same page about a really important thing!!!!!
(Merrin is very briefly tempted to be embarrassed about it, and catches herself, because if her first response to successfully noticing the really important thing is a negative one then she will implicitly be punishing her brain for having thought of it, and she knows better, and she has significantly more important priorities here than feeling stupid.)
"My suit is very well designed for what it does," she agrees. "And - I absolutely wouldn't have tried that without [healing spells] and someone to rescue me," and, you know, the fact that she still feels like the default outcome is dying trapped on an exoplanet, which shifts her degree of risk-aversion significantly, "but - I think I'd've put 90%, maybe 95%, odds of being fine if I hadn't been dragging you, because I just wouldn't hit anything? It sounds like you were modeling me as having less situation-relevant skill than I do, and just more generic physical toughness, but - I wasn't injured when I went in after you the time you were unconscious, I have a lot of training in more or less that exact thing, actually, rescuing injured people in hostile water conditions, that might be the single hazard-scenario I've trained on the most. I just wasn't used to the hydrodynamics of pulling you on a rope and failed to compensate."
Subtext: Merrin does NOT think the correct update for him to make here is that she's strictly less capable than he thought! That seems like the wrong update, and one that might incorrectly lead him to rule out plans that would in fact work. Also it would make her sad, she doesn't want him to feel like she's less capable than he thought she was and be disappointed in her for not being his Merrin-version
In the back of her mind, she's quietly making a mental note that Estha seems pretty thoroughly surprised and alarmed by the revelation about her normal human biology, whereas to Merrin it's entirely unsurprising and just obviously how it would work in an alternate-universe scenario.
(She's still not saying anything to him about that, apparently. Because she's uneasy, for reasons she can't pin down but that Laeirthe thinks she should provisionally listen to even if she can't lay them out in logic - and, like, he's not wrong that her judgement is constantly working like that, she often has a really hard time verbalizing the logic behind her intuitive judgements, it's super annoying but it doesn't mean her judgement is wrong in those situations - anyway, she...somehow feels like it might destabilize something, here, to push back too hard on alternate-Estha's current hypothesis that she shares some kind of meaningful continuity of self with his Merrin. And since there are all these things neither of them wants to talk about, if she does destabilize their current working relationship, she's not sure that talking about it longer would even fix it.)