Water leaking into the passenger side floor board.

I am thinking the heater core.

Tar Heel Person:if it does not smell sweet and is clear in color,it's most likely water from the A/C condensation drain line that is clogged.The drain is located directly below the center console. There is a drain hose there and this is where the water drains .You can clear it out from under the vehicle with compressed air or a coat hanger.if you can hear water sloshing around when turning left or right,maybe a gallon of water will come out when line is cleared.also change your cabin air filter.

Water appeared on passenger side foot well. It was not antifreeze but AC condensate. It is likely that when replacing the cabin air filter some debris dropped onto the fan below and later got blown into the AC condenser area thus plugging the condensate drain. The drain is a 1.5 inch black hose pinched at the end which protrudes towards the front of the car horizontally from the fire wall. Water drips from here down on car components below and to ground. It is located 6 inches below the refrigerant return line. It is visible and accessible from the passenger side of the engine compartment. Use a 4 inch piece of wire inserted into hose to clean it out and start the water draining. For a more thorough cleanout, remove the blower fan located below the glove box and above a horizontal plastic panel which is easy to remove. I found some dried leaf pieces in the blower. Take compressed air and blow back through the discharge hose to remove any remaining debris. This might also hold for other Mazdas e.g. Mazda 3