Learn Swedish: Vi umgås varje dag och vattnar varandras blommor. - We socialize every day and water each other's flowers.

It is the reciprocal pronoun varandra (each other) with the possessive -s attached to the end. Just like adding an apostrophe-s in English, varandra becomes varandras (each other's).

Why is it varandras blommor and not the definite form varandras blommorna?

In Swedish, a noun that follows a possessive word (like varandras) must always be in the bare, indefinite form. Even though you are talking about specific flowers, the possessive -s essentially replaces the need for a definite ending.

The verb umgås ends in an -s. Is this a passive verb?

No, umgås (to socialize / hang out) is a deponent verb, often just called an s-verb. This means it always ends in -s but has an active meaning. It is a standalone word and not the passive version of another verb.

Is it normal to skip the subject before vattnar?

Yes! Because the subject vi (we) is exactly the same for both actions in the sentence, you don't need to repeat it after the conjunction och (and). This works exactly like it does in English.