A fishing knife can be used for many different things out on the open water. Aside from filleting fish, you can use the knife to cut fishing line, ropes, and fruit (if the blade is clean and fish-free). For any simple tasks out at sea that may require scissors, you can use a fishing knife to complete.
If you are a hunter, you can reap the benefits of an ultra-sharp fishing knife by using the knife to trim fat from small game animals or birds.
No matter how you plan to use the knife, make sure to keep it out of reach of children and always bring the sheath. The sharp blade can take anyone by surprise and can slice human skin easily.