Similarly, in English you can speak about, for example, several people's faces. In Hebrew one person's one and only face is in plural form, הפנים שלו יפים. The word פן in the singular form does exist, it's not very frequent and means "aspect". But one's face is a plural. שמים, מים don't have a singular form.

Well, as AlmogL said, it's not "faces", it's "aspects". "His aspects (the aspects of his face that give it character) are beautiful".

That's seems very strange. Water I can understand because even in English it's more or less a mass noun but faces? How do you talk about multiple faces? פינימים?

In English it can be plural (even when speaking of a single amount of water, like the waters of the Jordan), in Hebrew, it can't be single. It's always plural.

It was used in England in Victorian era and earlier more frequently but it's still used today for spas/mineral water bathing. "Taking of the waters." Big thing for upscale women of a nervous disposition.... Or most things... medicine wasn't very advanced. Corsets were tight... Etc etc.

As others said, it's grammatically plural in Hebrew, so when you translate to English you need to adjust the way you think of "sky" and "life".

There's no other plural form, life is חיים and lives is also חיים.

Well, to mark them as plural, you can add an adjective like פָּנִים רַבּוֹת many faces.

With regard to פניה/פניו - this is actually exactly the plural possessive form (the singular form would be פנו/פנה), so there's consistency: פנים is indeed always plural when referring to someone's face.

So this would be singular or plural? (Thanks!) פנים אחרות

Well, פָּנִים אֲחֵרוֹת is primarily a different face, only in specific contexts where it is clear you encounter many people, you would read is as plural: אני מביט לתוך הקהל הזה, ואני רואה פנים אחרות I look into this crowd, and I see different faces.