My garters love trout strips, even dusted w/some calcium. Not sure if water snakes feeding cues are similar enough to ensure that they'd love trout filet as well, but it's worth a try.
:Have you tried trout strips? Local grocery should have fresh trout, slice into strips and store in freezer bags at 4(?) peices per bag. Thaw and feed. Costs about $3.00. :Feed goldfish for variety (provides whole food/calcium). :Remember, variety like guppies, etc will off-set deficiencies.