Well, I just found out the difference with an onsen. Basically, the onsen uses a natural hot spring water, while a sentou simply uses heated tap water like the one you use in your own house. They're both for bathing, but an onsen has the "natural hot-spring water" as a selling point. In practice, they're used for the same, but in an onsen you'll see those rocks used to make the bath itself often, while a sentou will literally be a big bath.