Evening,
“Please,” he said, quietly, and this time there was desperation in his voice. “Please, I’m lost and there’s nobody else to help me.”
She crouched down so that her face was level with his. This close she could see the shadows of his eyes, the lashes that clung together with the water. Dark hair close to his skull, the ends skimming the nape of his neck and falling down over his forehead. “What are you?”
“I’m a merman,” he answered.
“Fine. Just so long as we’re clear on that,” she said.
Oh. Good! Good, good! Nice flow, nice tension in the scene. What we need is a bit more precision in descriptions.
She crouched down so that her face was level with his.
One can’t crouch up. 
Also we might be able to nuke “that”
She crouched so her face was level with his.
Moving on,
This close she could see the shadows of his eyes
Shadows of the eyes sounds odd. Shadows in his eyes? I’d like more about the eyes. What color? Predatory or pleading? Invisible because lost in the shadows? Throw us a bone here.
Dark hair close to his skull, the ends skimming the nape of his neck and falling down over his forehead.
Not quite sure about that dark hair close to his skull… Is it short or just plastered with water? Do we really need the “close”?
Dark hair glistening with moisture, the ends skimming the nape of his neck and falling down over his forehead.
or
Dark hair skimmed the nape of his neck and fell down over his forehead.
The author is almost there. Just need to dial the detail up a bit to make the writing pop. He is all wet. Is that ocean water? Does he smell like the sea? Does his skin look odd? The other has drawn a good sketch. It just need fleshing out.
Optional critique:
If it was me – and this is the part where I take the author’s original style and chuck it out the window – I’d go more direct. We already know she is looking at him and the reader is concentrating on him. We don’t need the second reminder that she is there.
Original:
She crouched down so that her face was level with his. This close she could see the shadows of his eyes, the lashes that clung together with the water.
Revised:
She crouched so her face was level with his. Shadows hid his eyes. His long eyelashes clung together with the water.
