The natural born alicorns are all late-bloomers, getting their cutie marks as teenagers at the earliest. Eventide is no exception.
 
Eventide got her cutie mark as a teen on a sailing ship with her mother Twilight and sister Dapple on a visit to a friendly foreign country. She took to sailing like a fish took to swimming, shadowing the sailor ponies everywhere and learning all she could. The captain in particular took a liking to her and indulged her curiosity. When the ship sailed into a massive rogue storm, Twilight and Dapple tried to aid anyway they could but their magic and the expertise of the sailor ponies were barely effective at keeping the ship from sinking.
 
It was during this storm that something clicked inside Eventide. She dug in and found her unique alicorn magic to be so similar to the maelstrom of wild sea magic around her. She reached out with that magic and the waves and storm clouds obeyed. She found her place as the Princess of the Waves.
 
In the years to come, she continued to learn all she could about sailing and strategy, joining the Equestrian Navy when she became an adult. She would rise to be an admiral and when airship technology advanced to be viable for military use, she brought that air power under the navy’s control. Most her efforts were simply keeping Equestrian waters safe from pirates and the like. There would occasionally be some upstart foreign power who threatened Equestria, but having a solid, notoriously well-run navy made quite a few would-be warmongers think twice.