"Most" can be used in different contexts and with different meanings. In the first sentence "most" is a superlative, meaning that there is no place more densely populated. In the second sentence it means "not all easily accessible water, but most of it". The word "most" can also mean "very". For example: "This question was most interesting." 😏 I hope this helps.

Adjectives with two syllables can form the comparative either by adding -er or by preceeding the adjective with more. These adjectives form the superlative either by adding -est or by preceeding the adjective with most. https://www.macmillandictionary.com/amp/dictionary/british/most_1 Here is the link to a dictionary with bref explanation.

Thank you for your detailed explanation @Evil Eye

It's an adverb in your first example. The reason you see the "most + adverb" structure is simply because "most" is usually creating a compound modifier, in this case a compound adverb ("most densely populated"). In your second example, "most" is functioning as an indefinite pronoun.

Thank you, @Charles!