If the water is from the cowl area, sometimes it is the drain hole is plugged. As Neil said, if you have the car on a level surface and pour water (slowly, as the design is not supposed to stop a fire hose) on the right side of the cowl vents behind the hood, the water should pour on you foot which to perform this "feat" is positioned almost perfectly where the water comes out. Right behind the front tire. If nothing comes out, the hole is plugged or you have a lot of holes in the cowl area!!
If water comes out where it is supposed to do the test again with someone in the interior laying on their back (I hate this) with the passenger vent door opened. You can kinda look up into the large opening that the air comes in. There is about a one inch tall lip all the way around this opening. You can reach up and feel it. Many times is rotted and lets water in. There is a cowl repair kit that allows the installation of a tall plastic lip to be installed with, of course the heater remove. Many times this will cure the leak!
This is not an air-conditioned car is it?
Also, if you do have a couple small holes in the cowl, they can be repaired with JB Weld metal epoxy. If they are totally rotten then you need to gut the dash and spend money.
Also, a windshield can leak. If your cowl test remains dry and it is not anti-freeze (sickly sweet smell as well as green) then lay in there and look up while someone pours water around the windshield gaskets.
Rob