How do you find the density of H2O- Water?

You didn’t define what phase the water is in. Water can be liquid, solid, or gas. The density of steam or vapor is dependant on temperature and pressure and should behave as predicted by the Ideal Gas Law at low densities anyway. The density of solid water or ice depends on its structure. Crystaline ice is less dense than water because water is a polar molecule that can arrange itself more compactly when in its disorganized liquid state. The matrix of crystaline ice takes up more room. Not all ice is crystalline, amorphous ice can actually be denser than liquid water. Common ice is crystalline though.

While the kilogram was indeed intended to be the weight of 1 liter of water it is defined as the mass of an international reference kilogram.

The entire metric system is slightly miscalculated. The meter was originally defined as 1/10,000,000 of the distance from the Equator to the North Pole through Paris but it turns out their measurements were wrong so we have the incorrect meter to this day. Now the meter is based on the speed of light in a vacuum which is less variable and more easily measured than the distance between points on the Earth..

Indeed you can consider 1 liter of water to weight 1 kilogram or one gram of water to occupy 1 cubic centimeter.

For the metric challenged there is the old “a pint a pound the world around.” This used to be a rhyme every child knew. A few years ago I was at the grocery store and the young clerk moving two gallons of milk onto the scale to get them out of the way was astonished how heavy they were. She asked me if I knew how much 2 gallons of milk weighted and I answered 16 lbs. She asked me how I knew that and I answered, “a pint a pound the world around” and she said that she had never heard that.