Like Snow on Hungry Graves

The emperor's illegitimate son is forced to take his half-brother's place and travel to an enemy kingdom for an arranged marriage. If the bride's family find out about the deception they'll kill the prince. If they don't, his guards have orders to assassinate him during the wedding ceremony to provoke a war. Along the way the prince meets a stranger who is oddly eager to help him get out of this predicament... on condition that he helps her hunt a sea monster.