You’re correct, it’s compound. And it is the most abundant. It’s everywhere.
Only on the surface. The 10 most abundant elements on earth: https://education.jlab.org/glossary/abund_ele.html Hydrogen comes in at number 10 with 0.14%. While Oxygen is number 1 at 46.1%. Since water is H2O water is way down there on the abundance list.
Water its just less dense than most of the things the earth is made of and floats to the surface.