I think it's a great idea. However you need to be aware that a water bed heater is a very powerful heating device. Just think of how much it takes to maintain 4 or 5 hundred pounds of water at 80*F in unknown conditions (cold rooms). The fact that it has a thermostat and a temperature sensing probe is a real plus. I would use a thermometer to watch how it heats the soil until confidence is achieved. Go for it. You probably already have.
I am also in Ohio, so I haven't started my seeds yet. I have a heater from our waterbed downstairs and started thinking about trying it. Just curious if anyone else had tried it before. Thanks!