Why would water accumulate on the driver side floor of Pontiac Transport SE 1996?

If it is just water and not coolant, then more than likely the A/C drain line is plugged. Crawl under the car, passenger side, and find the drain line near the firewall and unclog it. If it is coolant then your heater core is leaking and needs replacing. Test the water and if it smells or taste sweet it is coolant.