There are 3 books in the Mer Chronicles series. The newest book is Immerse which came out in 2019.

1
Emerge

Book cover of Emerge
Lia Nautilus may be a Mermaid but she's never lived in the ocean. War has ravaged the seven seas ever since the infamous Little Mermaid unleashed a curse that stripped Mer of their immortality. Lia has grown up in a secret community of land-dwelling Mer hidden among Malibu's seaside mansions.…
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2
Submerge

Book cover of Submerge
Lia and Clay’s love has broken the Little Mermaid’s curse, but their ever after may not be as happy as they planned. Lia is adamant about staying on land with Clay for her senior year despite her family’s opportunity to move to the new, sparkling capital city below the waves.…

3
Immerse

Book cover of Immerse
The descendants of the Little Mermaid face deadly magic, shifting alliances, and the dangers of forbidden love in this final installment of the Mer Chronicles. Lia can’t wait for her parents’ coronation. Now living in the sparkling palace beneath the waves, she sneaks off to Malibu whenever possible to see…