On intrinsic value:
Both of the goods you mention (water and oxygen) have unique properties that are intrinsic, but when it comes to value, it necessarily introduces a market.
Markets are external to the goods and determine how to value the good, not the good itself.
Consequently, there is no price floor for any good that is determined by the good itself and valuation will always be the result of supply and demand.
Intrinsic value vs instrumental value. Almost every "good" is priced to its instrumental value. In the case of air and water it is their life giving properties and relative scarcity (or abundance) which drives the price someone is willing to pay for it.
Something that has only intrinsic value is much more rare and difficult to price. These are items that serve absolutely no purpose whatsoever and are only valuable insofar as they exist. Like art
