I hear water , running back and forth in or rear of the driver seat of the extended cab area

Do you see water or are you hearing gas sloshing around in the tank?

I hear water and some water comes to under my seat

Like FoR said, that's where the gas tank is on a Mazda pickup. The noise might have always been there (baffles will try to limit the sloshing) but you might have lost some sound insulation in the cab over the years.

Well that changes my answer then, you may have a leak in your cab then and water is pooling in the floor pan area

Clean the water up and check all the door and window seals and also the door drains.

Rags and paper towels should work for clean up. You may want to pull up the carpet though.

I will take the floor carpet out to see where the water is coming from, and then will post all I learn from this. Thanks to all you have input to this . Thanks to you all again

Was it water in the door from a car wash?

What was the end result? I have a similar issue and am unable to find the cause.