Elsa isn't equipped to do that and it looks like she's just training. Why? I have no clue as she was insanely powerful in the first movie, but freezing the ocean is a terrible idea in storm as you'd have to freeze so much water around you so as not to be crushed by moving ice that you'd freeze your ship in the process.
I mean, them finding their parents makes at least some sense, but they seem to just be wandering through the wilderness so I don't think thats it.
Its based on Hans Christen Andersen's Snow Queen story, but only very superficially, as there is no troll or magical mirror and those are kind of important.
Really there's just a bunch of random images with very little context. Also I don't like her training. It just feels...really specific. Like, why is running on water with ice something you're training for? Just freeze the bloody thing and walk on the walkway you just made. And refrain from doing it on a storm tossed ocean.