You can't have a proper beachside cookout without some fried catfish!
In a small mixing bowl, mix the buttermilk, water, salt, and pepper. Pour that stuff into a flat pan that's big enough to hold the catfish fillets. Spread those fillets in a single layer over the bottom of the pan, turning them over to get an even coating. Put the pan aside so the fish can marinate.
While the fish is doing its thing, combine the cornmeal, flour, and Old Bay in a 2 gallon resealable plastic bag. Add the fish in there, a few fillets at a time, and give it a gentle shake to coat evenly.
Heat up your veggie oil in a deep fryer to 365 F.
Deep fry the fish until they're golden brown, about 3 minutes. Careful not to place them too close together, they need to be able to brown. The fish should be slightly crisp outside, and moist and flaky inside. Take the fillets out of the oil and let them drain on paper towels.