Take a look at the passage again:
Marina, the beautiful mermaid, wanted some
tuna salad. But had a small problem since she was allergic to celery. At
Sammy’s Sub Shop, Marina hoped to find tuna salad free of this dangerous
vegetable. Flopping across the tiled floor to the counter. Marina placed
her order and then checked her sandwich for celery. Not noticing, however,
the spoiled mayonnaise. At five o'clock that evening, Marina became violently
ill with food poisoning. When a lifeguard at the beach discovered the problem,
he called 911. Even though the mermaid had fishy breath. A handsome paramedic
gave her mouth-to-mouth resuscitation. Wailing like a sick dog, the ambulance
sped off to the hospital. Where the doctor on call refused to treat a sea
creature with a scaly tail. A kind nurse, however, had more sympathy. After
she found some Pepto-Bismol. Marina drank the entire bottle of pink liquid,
feeling an immediate improvement. The mermaid told the rude doctor never
to swim in the ocean. For she would order
hungry sharks to bite off the doctor's legs.
While sharp-clawed crabs plucked out his eyes.
Tossing her long hair, Marina thanked the nurse for the Pepto-Bismol. And
took a mint from David, the handsome paramedic.
A fragment will not contain a main clause. A main clause follows this pattern:
Subject + verb = complete thought.
In the highlighted item, she is the subject, and would order is the verb. This pair makes a complete thought, so you do have a sentence.
For is a coordinating conjunction in this sentence. Although some teachers frown on using coordinating conjunctions like and and but to start a sentence, no rule actually says that you cannot begin with one.