I feed my baby watersnakes in small tupperware containers. I put the fish in there with only about 1/4 inch of water. It makes it quite a bit easier for the snakes to catch them.
In the wild, watersnakes probably don't nab too many healthy fish from the water unless they are confined to very shallow waters. They probably rely on weak and dying fish for a good part of their diet. Watersnakes use a combination of scent and sight for hunting but it probably varies with the situation and prey.