You can try earth worms. I'm not sure if baby waters eat worms like garters do but they are both very similar species. ou could also try tadpoles. They are big on those to. Its your best luck until he reaches a size where you can try switching him onto pinkie mice. The guppy like fish you caught are probably mosquito which are fine for him to eat, they are the North American cousin to the guppy. You can get him fish at the pet store to, avoid goldfish and rosy red minnows though. Also you can give him pieces of a fish fillet from the supermarket, salmon, trout, ect. Good luck. I've been trying to fine myself a little watersnake for a long time!