Why is the density of water important for life?

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.
And water pistols would essentially be laser guns.