Accented syllables are often very subtle and picking them up requires careful listening and sometimes a friend or teacher to correct you. In the case of みず, neither syllable is particularly accented, but inflection can change a lot based on the emotional state of the speaker.

It just takes practice. The rules for accents and inflection are not written, to my knowledge. I recommend watching a lot of Japanese TV.

There are, but like with anywhere, they can vary depending on location and dialect/accent. The best thing to do b is watch Japanese TV and movies - you’ll end up with a Tokyo intonation and accent, but it will be consistent, which is the important thing.

Try https://www.gavo.t.u-tokyo.ac.jp/ojad/search/index/word:%E3%81%BF%E3%81%9A

Imbortant to note, the accent in japanese is not stressed like english or french, but pitched. So when you are listening for it in your duoligo, don't listen for the syllable with attack, listen for places where the pitch goes up or down.

there are 4 patterns of pitch accent. A good explainer is Here, or on youtube from Dogen

みず is あたまだか, where the first mora is high, then the second one (and all the others would be) is low. You can hear the fall on the ず, where it geos from the starting pitch to a lower one