Space is a vacuum because it has water in it? That’s not how that works. Vacuum is the absence (or near absence) of physical matter. In interplanetary space there are about 5 atoms/molecules per cubic centimeter. In interstellar and intergalactic space this density decreases even further. For comparison a cubic centimeter of water on earth has about 3.3 E22 water molecules.

Also, the Bible doesn’t say that the earth is flat or stationary. It says that the Earth’s foundations are firmly set. The foundations of the earth is the boundary where the crust meets the mantle (known in geology as the Moho discontinuity). From the perspective of people living in biblical times, this was meant to convey that 1. The earth is mostly stable, 2. That God makes the earth stable, and 3. That despite various natural disasters the Earth will not be completely destroyed like in the Flood again (at least not until the end of the Millennium when the New Heavens and New Earth will replace the former things).