I have no idea about waterbed heaters. I am trying using a work light about 18" above my starter pots...1/2 the seeds germinated the other 1/2 haven't... yet...

I sure wouldn't use it. You have no way of controling the heat you get. My heat mat feels barely warm when I put my hand on top of it. Too much heat and you'll kill the seeds not to mention start a fire. Before I got my heat mat I didn't use anything and I still don't for many seeds. Only those that require bottom heat for better germination. Even without the mat I had pretty good germination with those seeds.

If you really want a heat source try a regular light bulb. Rig some kind of open structure like a frame made with scrap lumber with a cookie sheet on top and put the light bulb under it. The heat from the bulb will transfer to the cookie sheet. You only want to keep the bottom of your pots warm not hot

If you have read through the previous discussions here about using water bed heaters then you know that it is the attached thermometer probe, the enclosure around the seed starting space, and the elevated spacing created between the pad heater and the bottom of the trays/pots that lets it work.

Not that I am advocating using one, there are many alternatives. But it can work and work safely with an enclosure, a 1-2" elevated platform over the pad, and a container of wet soil sitting on the platform with the thermometer buried in it to trigger the mat off and on. You are heating the air in the enclosure so only indirectly heating the flats of seeds.

Dave