I'm currently writing a fanfic where Sea Swirl is a recurring character, but I haven't given her any characterisation beyond being Sassaflash's seriouser friend because I don't know what to do with her. Her cutie mark is two dolphins, but what destiny could that represent?

6980591
I don't think seriouser is a word.

Anyway could be sea surfing or diving. I am not familiar with any dolphin symbolism, just know that they are smart and playful (or at least appear to be happy and playful). Heh could go with marine biologist since I bet you can say that she was in Celestia's School for gifted unicorns like all other background extras.

She could perhaps be good at handling marine life and be able to communicate with dolphins, sharks, fish, etc.

6980607
Hmm... that last sentence gave me ideas for what she could do in a sequel.