If water had no density - if its mass per unit volume were zero - it would be massless, and would travel at the speed of light. The oceans would all go in all directions (leaving sea-dwelling flora and fauna behind - and all the salt). You wouldn't see rain coming (at least, not directly at you), because the image of the rain would travel no faster than the rain itself. <span>And water pistols would essentially be laser guns.</span>
