Is that to say it's impossible to destroy?

no, its very easy to destroy. it can react with metals to form hydrogen gas and/or oxygen, you can run a current through to get the same. many organic reactions will take up water too.

we can get it back though, hydrogen burns in air to give water, respiration take oxygen from the air and hydrogen from fuel to give water as a byproduct.

i wouldn't say its technically correct that water is the same since the world was formed, but i would imagine the vast bulk is.

Matter can neither be created nor destroyed, only changed, as RAOVQ has already pointed out.