In Rising Tides (Tidal Tithe #2) by JA Townsend:
 
Forced back into the sea, Lyra returns to an Atlantis she barely recognizes. New rulers hold power, old loyalties have crumbled, and her family name now carries dangerous weight. Bound by a bargain with the High King, she must fulfill the first part of the pact beneath the sea—but Atlantis does not welcome change, least of all from a mermaid.
 
Political unrest swells through the kingdom like a rising tide. Betrayal hides in every shadowed current. And the greatest threat to Lyra’s life is time itself—the bargain will claim what is owed.
 
On land, Wilder faces an impossible choice. The High King refuses to wage war against Atlantis, leaving Wilder with only one desperate path: challenge for a throne he never wanted. But when a betrayal of staggering proportions tears his plan apart, Wilder finds himself further than ever from saving Lyra—from the sea, and from the bargain slowly draining her life.