Okay, so this is a surprisingly specific and awesome request. I’ve been down this rabbit hole. You’re looking for emotional depth, which to me means the story has to make you feel the pining, the loneliness, or the connection between characters, not just have a cool underwater setting. A lot of mermaid BL can skew a little silly or pure smut, which is fun but not what you want.
My absolute top recommendation would be 'The Merman's Song' by D.C. Morin. It’s an indie title, but the audiobook narration is incredible—the narrator does this ethereal, watery quality to the merman’s voice that just pulls you in. The story is about a lighthouse keeper and a merman who’s lost his pod, and it’s all about grief, found family, and quiet intimacy. It’s more slow-burn and atmospheric than action-packed.
Other places to look: the audiobook for Kai Butler's 'Ocean's Echo' has a merman side character whose storyline is heartbreakingly beautiful. Check the LGBTQ+ romance sections on Audible and Scribd with keywords like ‘siren,’ ‘mer,’ and ‘aquatic.’ Sometimes you’ll find gems tagged under ‘fantasy romance’ or ‘paranormal’ that aren’t specifically in the BL category but absolutely are.