Okay I'm back! Also, the whole love thing seems fun. But I'd make the plot a bit more larger. Again, i'm going to use Troy as an example.

A war has a reason and a cause. The reason is what's filling the bucket up. For WW1 the reason was a mixture of hate towards several countries as well as vying to get more land. The cause (or trigger) was the death of the crown prince of Austria-Hungary. If he didn't die, someone else would have within 3 years and there still would have been a war.

With Troy it was sort of the same if you look passed the legend. Greek had to pay taxes to go through Troy, who was strategically placed at the enterance to the black sea. Greece long wanted this position for itself. The kidnapping of Helen was just the trigger for this.

I'd suggest to do the same with the mermaids. The princess is tricked into kidnapping the human prince. Because some evil advisor of the mermaid king advises her to do so. Of course humans become furious and attack, giving the advisor (who might also be the leader of the troops) a reason to retaliate and start conquering the shores.