Learn Croatian: Ako se brtva raspukne, komora će se brzo napuniti vodom. - If the seal bursts, the chamber will quickly fill with water.

This is the anticausative reflexive construction you are focusing on in this lesson. By adding se, we show that a change of state happens spontaneously, without a specific agent causing it. The seal bursts on its own (se raspukne), and the chamber fills up (se napuniti) as a natural consequence, rather than someone actively breaking the seal or filling the chamber.

Could I say komora će brzo biti napunjena vodom instead?

You could, but it changes the focus. Biti napunjena is the true passive and implies an external agent, hinting that someone or some outside force will actively fill it. Using se napuniti focuses purely on the spontaneous process of the chamber filling up as a result of the leak, which sounds much more natural for a mechanical failure like this.

Why is it just vodom (with water) instead of s vodom?

In Croatian, when you use the instrumental case to indicate the means, instrument, or material involved in an action (in this case, the substance filling the chamber), you must drop the preposition s or sa. You generally only use s or sa when talking about company, like doing something alongside a person or another object.

How does raspukne differ from just pukne?