Sea and Sky by Morgan Lysand

After twenty years bound to a pirate captain, merman Kyler is desperate for his freedom before his scale consumes him. For reasons unknown to Kyler, the captain agrees to set him free, if he can pay the steep price by the end of summer.
Prince Gavin, third son of the king has been denied the Grand Mage status he’s sought for seven years. Finally, the king decides to grant him a Task that would lead to him receiving the recognition he deserves. Unfortunately, there’s a pesky arranged marriage he has to deal with.
When Kyler saves the charming prince from drowning, Gavin asks him to play his lover for a month to get out of marrying a princess from another land. Kyler agrees after Gavin offers him a favor. A favor Kyler intends to take full advantage of.

Will Kyler gain his freedom or will the captain keep him forever? Can Gavin protect his heart and become a Grand Mage or will he be torn apart by betrayal?