Here's an idea for a story:

Sweetie Belle has never seen eye to eye with life underwater. However, just after she and her friends solve a friendship problem, she accidentally lets her criticisms about the ocean slip, offending the seaponies & sea creatures and getting into trouble.

As a punishment, Queen Novo says that the filly will temporary live under the sea until she understand that life underwater is just as good as life on the land.

Ocean Flow takes the unhappy filly in, and takes her under her wing.

At first, Sweetie Belle is miserable, feeling really guilty about her poor choice of words about the ocean, however, as time goes on, she starts to enjoy herself and even develop 'mother and daughter' relationship with Ocean Flow.

When the times comes for her punishment to be up, Sweetie Belle doesn't want to leave. She askes if she can stay and live with Ocean Flow (Who she now calls Mommy). After getting permission from her original parents (Cookies Crumbles and Hondo Flanks) and big sister (Rarity), Sweetie Belle is soon adopted by Ocean Flow who is more than have the filly as her official daughter.

Sweetie Belle soon settles in nicely into her new home and gets on well with the other seaponies who now have grown fond of her. The little filly still visits her friends and original family in Ponyville, and they likewise visit her. She even visits her new father, Sky Beak.

Years later, she becomes at beautiful seapony just like her mother.