Fish tends to be very wet, and not just because it lives in the water. Often it's frozen or, if it's fresh, it may still be packed on ice to keep it cold. Both of those methods add a lot of excess water to the fish that you'll need to deal with before cooking. Frozen fish especially will release a lot of water as it thaws, and it needs to be properly dried before it's breaded.
With just a few basic ingredients, onion rings are an easy dish to throw together when you want some home-made fast food. But getting that crispy texture right can make or break your snack. If you find you're not getting the crunch you want with your home-made ones, turn to your pantry for a box of cornflakes. This cereal will stand in for breadcrumbs in a traditional coating, but offer a more craggy, crunchy texture.