Kin no Kuni Mizu no Kuni

Synopsis

From Seven Seas: Once upon a time, two kingdoms, one rich in gold, the other rich in water, shared a border. The two nations were always fighting until an agreement was struck: the most beautiful girl in the gold kingdom would marry the smartest man in the water kingdom, binding the two nations together. However, both sides tried to renege on the deal by sending animals instead! Sarah, a low-level princess, is selected to wed the man from the water kingdom, but is sent a puppy in place of a groom! Meanwhile, the candidate from the water kingdom, Naranbayar, is saddled with a kitten! When Sarah and Naranbayar meet, they decide that, in order to stop a war between their countries, they will pretend to be in love. Will their plan work? And what if real romantic feelings blossom between them? Note: Nominated for the 10th Manga Taishō in 2017.

Synopsis & details from MangaUpdates