Assuming you are talking about the engine fresh water system, my advice is "Don't do it."
With no water in the engine fresh water system the engine will not be cooled at all, and will overheat probably within a matter of ten minutes or so resulting in seizure and distortion, and a very large bill for engine repairs or renewal.
If you are unsure of the water where your boat is kept, then fill some containers with fresh water from home and take them to your boat in readiness for when you need to refill your cooling system.
The raw water pump will take suction from the water your boat is floating in, but it only passes that water through the heat exchanger/exhaust manifold before it is discharged into the exhaust system and then overboard. The fresh water cooling passes through a separate part of the heat exchanger where it is cooled by being in contact with, but separated from the seawater section of the heat exchanger/manifold. But if there is no water in the engine fresh water system then the rest of the engine will not be cooled, and it will overheat leading to seizure if you don't stop it fairly smartly.
I can't stress the point too much, don't do it.
If you are only worried about having no water in your drinking water tanks, that is not a problem so long as you don't get thirsty.