In most works of fiction, Atlanteans are weakened while out of water. This includes DC Comics' Atlanteans. As said in the comics' description of Mera on the DC Comics wiki :
Prolonged exposure in an above-water environment will cause Mera to weaken, and ultimately die.
On the linked page, this weakness is linked to Mera's peculiar metabolism, which is wired to breath underwater, not above ground. Still, quickened dehydration is a common trope when dealing with Atlanteans, which follows the same rules as the paragraph above. Take for instance Namor's description on the Marvel wiki :
When not exposed to water, many of Namor’s physical attributes gradually decline and can reach a point where he possesses only a small fraction of his powers. Namor's strength, stamina, and durability are affected most and, at their minimum known level, are much closer to the levels possessed by the vast majority of other Atlanteans. Renewed exposure to water will immediately restore his abilities.
In short, we could explain Mera using Aquaman's own bodily fluids as her water source to maximize her own "wetness" and push back her potential dehydration. It is true that, unlike other Atlanteans characters depicted in the movie, she doesn't need a special apparatus to breath our atmosphere or withstand dehydration... But at the time, they were stuck in a dusty temple smack-dab into Sahara, so it makes sense she'd want to protect herself, while Arthur as a half-human would be less vulnerable to typical Atlantean weaknesses. She wouldn't have explicitly told Arthur she was at risk because she was somewhat prideful and refused to look weak before him.
Or perhaps I'm reading way too much into this, and Mera simply cannot manipulate her own bodily fluids. Either way, I have not found an official statement regarding this matter, and thus can only speculate using common tropes and the movie's events.