Around here, "rainbait" is usually bay anchovies, but I think is is a generic term for small baitfish that splash the surface, and yes, you can get them with a small (1/4 square mesh) castnet. Bubbles and and Rattlesnake hold freshwater fish like trout and perch that came down the pipe from the Connetquot river and get trapped in the brackish pond. Plus in the late fall/winter, I've seen bunker which came in from Great River wind up dying off there.