I have my little broadbanded watersnake in a roomy snake condo that we built, with a pane of glass you slide off the front. he's roommates with a now very large ribbon snake. I recommend minnows, I catch mine in a ditch in front of our house with a fine net. about 10-20 minnows 2-3 times a week. Give him a branch to climb on, and a large water dish to sleep in, and he'll be fine. just be careful that the whole cage doesn't get wet, or he may get bubbles and blisters under his skin and die. I learned the hard way once.
put newspaper in the cage for easy cleaning. Good luck with your new little friend!