I've suggested a few tweaks, Julie, for emphasis or occasional correction. Rick has made good points about physiology. The details you already have are very evocative for characterisation and background. It's an excellent opening to a story.
As you bring in Sicily later in the scene, when I read this a second time I felt you needed an indication nearer the start that this is a real-world setting and then say (this can be left until a bit later) roughly where in the world Lorna is. Otherwise, for me, the reality of Sicily comes as a surprise and I started wondering exactly where Lorna was.
Lorna flipped her webbed toes and snuck silently through the underwater cavern, her skin lighting the darkness. Perhaps say she has arms. I know toes implies mammalian body structure but I didn't form an instant vision of her. You could mention her gills too - readers of any age like to know how someone breathes underwater - and say they're under her arms. Then later with the boy you could remind the reader that the boy couldn't see her gills as they are tucked away. Rock walls widened into the open sea. The current urged her on, pulling her long braid out in front of her as Rick says, the braid would be flowing behind her and putting a smile on her face. There’d be trouble again when Mochin found out, but farming lessons were such a bore. She’d choose adventure over safety, any day. I think a hint of what sort of adventures she has would be good here to show up her character and background. At present the last piece of information finishes rather abruptly. Are her adventures the sort of general exploring anyone with an adventurous spirit might do or has she, in the past, gone out on specific missions?
The sea floor swayed with spinak, and despite her people’s best efforts, its leaves were dull and dark instead of their usual shiny purple. She gently ripped a leaf off and munched, following the noise-that-never-stopped. The
offshore oil rig ground meaning of ground ambiguous? Not obvious enough from syntax here that it's a sound. ?The oil rig bore down through screaming rock day and night, pumping life’s precious nutrients up to where the humans could steal them. You could make more of screaming by putting it in its own sentence. I'm guessing you can't hear submarine drilling from the surface. If that's true, maybe something like you couldn't hear it on the water's surface (implying the human world) but Lorna hears the screams when she dives. That could also make the submarine atmosphere more vivid and delicate. Humans were to blame for her people’s sickness, but constantly hiding in the city-caverns city was a bit of a jolt as I'd previously imagined the cavern to belong to a rural coast didn’t help either. I don't write for children but is the wording 'constantly hiding ... help either' too adult somehow?
Light filtered through shades of blue as she swam upwards. Only her eyes broke the surface Rick's right about her head physiology not being clear. ?Only the top of her head and eyes broke the surface? and the gray would the sea have shades of blue if the sky is gray? sky helped camouflage the glimmer her skin gave off. Men in bright orange suits scurried about the oil rig far above, too busy to notice her. The boy was in his usual place, fishing from a boat beneath the pillars that supported the immense show rather than tell here, e.g. use the size of the boy to compare with the tall legs of the platform with its cranes and tower balanced on top, or whatever your oil rig looks like, . She paddled closer.
As the only child born in decades, she often spied on the boy. He too seemed to live in a world of adults. Sometimes she thought she could feel his loneliness, but it was silly to think humans had feelings. As though to prove her point, the boy carelessly threw a half-full plastic bottle of dark liquid into the waves.
Rage bolted through her slim body. She pounced on the floating object with its red label and lobbed it back. “STUPID BOY, KEEP YOUR POLLUTION ON LAND!” She pushed her thoughts out, honing in on him like a lightning rod.
The boy’s eyes widened, and his head rocked back as her thoughts hit their mark. Rising in fear, he startled again as the bottle reached him a second later would it take as long as a second?. It was enough to unbalance him, and he toppled into the sea. show a bit more here. A splash and the calm water swallowed him?
Lorna had never connected so quickly to another being not of
their her race. It was unexpected that the boy could hear her. She frowned. Why didn’t he surface? She let herself sink underwater and swam closer. An old fishing net wrapped itself around the boy's leg and with each frantic tug he made to pull himself free, the net held him tighter tighter around the boy’s leg with each frantic tug.
She smirked. Served him right. Forgotten fishing nets had trapped and killed too many of her whale and dolphin friends. As she swam up next to him, the boy stopped struggling and looked her in the eyes. Fear and bewilderment filled his sea-green gaze. Would it be green? Stick to grey? (an aside but is sea water around oil rigs green? In my ignorance I associate green sea water with a chalk seabed rather than rock.)
Shame kicked her into motion. She hadn’t meant to hurt the boy. Too bad she didn’t have her backpack. Her teeth would have to do. “I’ll free you.” She couldn’t stop to see if he caught her thought this time, but he held still while her molars cut through the net. The boy didn’t have much time left. He needed air. Her own Heart beating fast, she pulled his leg out and dragged him to the surface.
His body hung limp in her arms. Polluted! What had she done? She hauled him into the boat and pounded on his back. Relief flooded through her as he coughed and sputtered. Moving to the farthest corner boats don't really have corners. She could move to the bow or the stern of the boat, she waited. He should be okay now, but it was best to make sure. Music from the boat’s radio mixed with his coughing. You could make more of the music as a pollutant if you wanted to. The mention of music here feels incidental - sorry for the pun - yet you need the radio for later on.
The boy wiped his mouth with the back of his hand and glared at her. “Didn’t know there were any of you left.” He studied her, gulping in ragged breaths of air.
In the platform’s shadow, she knew her skin shone with a silver glow, and her eyes were larger than his, making it easier to see in the murky depths of the sea. Other than that, he shouldn’t find much to gawk at since her gills were hidden under her arms. Again, she connected her mind to his. “I’m Lorna,” she offered.
“How are you doing that? Can you read my mind?” His tone was curious rather than accusing.
“Of course not. I can only reach out. The rest is up to the receiver.” She bit her lip. Although her people thought in all the major human languages, no one had ever been able to establish communication.
“Ricki Ferrari. I…” he broke off as an excited voice on the radio interrupted.
“News flash! A glimmermen spaceship has been discovered off the coast of Sicily. It looks to be intact and scientists are flocking to the site to study this amazing discovery.”
Lorna tensed. An undamaged spaceship? This could be the answer to their problems. There was only one way to find out if it was an old colony ship or the long-awaited mothership. What was she doing sitting here talking to the enemy while her people suffered? Throwing a last glance at Ricki, she dove into the sea.