Final answer:
Frederick Marryat is the British author frequently viewed as co-founder of the nautical fiction genre alongside James Fenimore Cooper.
Step-by-step explanation:
The British author who is often considered alongside James Fenimore Cooper as a co-founder of the nautical fiction genre is Frederick Marryat. Marryat was a Royal Navy officer and a contemporary of Cooper, and he penned pioneering nautical novels such as Mr. Midshipman Easy and Peter Simple. His experiences at sea provided authentic details that contributed to the development and popularity of the genre in the 19th century.