Concept: teenage mermaid struggles to convince her parents that she’s not, in fact, hanging out near the surface all the time because she’s involved in a forbidden romance with a human; it’s just that her smartphone is only waterproof to a depth of two metres.
Mermaid youth culture is basically forced to piggyback on human social media networks due to a lack of undersea telecommunications infrastructure.
(Unfortunately, that isn’t likely to be remedied any time soon, because the mermaid ruling class are a bunch of reactionary technophobes who still regard metalworking as suspiciously newfangled. Remember all those news stories about sharks attacking undersea fibre-optic cables? Actually mermaids.)