If you're going on a fishing trip run by a fishing charter, you might be able to buy snacks on the boat, but it would be a better idea to bring some of your own. That way, you're prepared in case everyone else gets to the charter-provided snacks before you do, and you won't have to worry about whether the snacks are something you like to eat. Because you'll have to keep the snacks with you, they should meet a few requirements when you choose them.
The snacks should not have any contact with the fish you catch or the water. Even if the snacks are sealed in packaging, do not place them in the same cooler you're using to store fish -- not even in a separate compartment -- and do not eat them with your hands until after you've washed your hands. You do not want to contaminate the snacks with whatever pathogens might have been on the fish. Just because the fish were swimming in water doesn't mean that you can grab them and not worry about potential food illness. Ensure all snacks are kept well-wrapped and are easy to store away from the fish.
The charter might have a refrigerator for its snacks, but it might not have room for your snacks in the appliance. Take items that do not require refrigeration. Energy bars, whole fruit, peanut butter, nuts, and more can provide the energy you need without requiring a lot of storage space. If it's a hot day, too, be aware of how that could affect the food. For example, if you decide to bring a separate cooler in which to store food, very hot temperatures will make it harder to keep the cooler cool for a long time. Even the best insulation will eventually let heat through on a sweltering day.
Bring water and other drinks, but please ensure they are not alcoholic. Kicking back with a beer on the lake while waiting for a fish to bite may seem relaxing. But on a fishing charter, you don't want to have your awareness blunted in any way, especially if you're on an ocean charter where there may be rough seas. You have to be as alert as possible.
You can call the charters you're interested in and find out what snacks they have, as well as whether there will be room for you to bring your own cooler if you really want refrigerated foods. However, they'll also be able to suggest unrefrigerated snacks that other customers have seemed to do well with. Fishing should be fun, so grab a few simple snacks and go!