Most watersnakes would happily devour frogs or lizards kept in the tank with them.
The only way I can imagine that working is with a watersnake that is a dietary specialist, such as one of the crayfish-eating Regina species, the salamander-eating Farancia abacura, or the eel-eating F. erytrogramma. Of course these specialized snakes are far more difficult to keep than typical water snakes, and are seldom available in the trade.
You also must consider that many terrestrial species are prone to drowning, so having a pool in the terrarium may not work well.