There's a few possibilities I can think of -
- a blocked evaporator drain line from the AC (you say you don't use the AC, but did the recently previous owner and it's still wet from that?). If you're going to school or work early on a humid Florida morning and using the defog/defrost, even on heat, that's running the AC compressor to help de-humidify (remove moisture from) the air.
- a damaged door or window (possibly windshield) seal that's allowing rain to sneak/seep in. I don't know enough about your model to know for sure, but it could also be a blocked drain that's supposed to be channeling rain water down and out the bottom of the car.
- someone spilled something and isn't fessing up
- you accidentally left a window cracked open and rain got in?
What makes you say it isn't coolant? In most cars the heater core is on the passenger side (also happens to be where the blower for the AC/heat is located). A small leak in the heater core could cause liquid to show up on the passenger floor. Very small amounts of coolant may not smell different from water (assuming the previous owner used coolant and not just water), but should feel slightly oily between your fingers compared to water.