An alternative solution you can use for windshield watermarks is ELEVO watermark remover. Works like magic! You'll hardly break a sweat. (On the other hand... chemicals this powerful will most likely have harmful effects to exposed skin, so be sure to wear gloves.)

If the watermarks are on your car's paint, then like *manny31671 said, bad trip na bad trip talaga yan.

Try to examine the watermark closely. Kung naka alsa, then pwede mo pa siguro matanggal yan with cleaner wax. Kung naka-lubog, baka water-etching na yan due to acid rain. If that's the case, then you'll have no other alternative but to use a polish to buff out the defect.