Jensen is a mermaid - one of the last. He's been alone and bitter for way too long, with only an equally sour and sarcastic dolphin (you choose who) for company. The friend likes to show off near a stretch of beach where disabled swimmers come to swim with him. When Jared turns up helped from his wheelchair, Jensen makes him his goal. He has an old shell spell to turn a human into a mermaid - and make him an ideal mate for Jensen.
Trouble is - it heals Jared's legs, and gives Jensen human form, without a voice, to stop his acid tongue, to make him Jared's ideal man. You choose whether dolphin friend gets a voice or human form too. (non-porny is fine but porny is good too)