Learn Spanish (Latin America): El agua es una necesidad para todos.

Yes, agua is actually a feminine noun! However, because it starts with a stressed 'a' sound, saying la agua would cause the two 'a' sounds to run together. To avoid this, Spanish uses the masculine article el in the singular. If you add an adjective, it must still be feminine (for example, el agua fría), and the plural goes back to being feminine with las aguas.

Why use para instead of por here?

You use para here because it indicates the recipient or the target audience of the necessity. When you mean "intended for" a specific person or group (in this case, everyone), para is the correct choice.

Could I use todo el mundo instead of todos to mean everyone?

Definitely! Para todo el mundo (literally "for all the world") is a very natural way to say "for everyone" in Spanish and means exactly the same thing as para todos in this sentence.

The current lesson is about phrases like es necesario que. Why don't we use que or the subjunctive here?

Because this sentence states a straightforward fact about a noun (el agua), rather than evaluating an action. The lesson's structure (es + adjective + que + subjunctive) is used when you want to say it's necessary that someone does something. For example, you would use it in Es necesario que todos tengan agua (It is necessary that everyone has water). In our sentence, we are simply linking a subject to a noun.