Elsa sets off both trailers by going toe-to-toe with a raging sea. There are a few ways this could been seen, with one such explanation being that this is a sort of training session. The queen could be working to see how far she can take her powers, since she's going against a literal force of nature. While Elsa gets bowled over pretty thoroughly in the clip, the scene shows a determination that alludes to preparing for a bigger battle. Sure, Elsa could just want to master her abilities, but it's more likely that something has sparked this desire to conquer another element.
Another explanation could be that Elsa is attempting to conquer the sea to get across it. She's obviously determined to get past it, so why wouldn't she be using it to get somewhere else? Maybe the water is blocking her from getting to someone who needs her help, like her sister or Olaf?
