There's not a big database where all the planets we know about are listed, and each one has an entry that says, habitable: y/n. So when a real-life scientist (scientists in fiction may be different) is talking about habitable planets, it's an unofficial, intuitive thing. One individual scientist might have strict criteria, but her rules are liable to different from the next guy over's.

The biggest challenges, as I see it, are gravity and atmospheric pressure. Gravity, because there's no good way to mimic it, and if you live in zero-g for too long, your bones get spongy.

Pressure, because if the pressure outside is too high or too low, a leak can become a disaster almost instantly. If the outside pressure is about right, but isn't breathable, and your habitat springs a leak, you can fix the problem with duck tape, and the actual repair can wait until it's convenient.

A world that was just like Earth, except covered in 100% ocean, would be a jackpot. We're already 90% of the way to having a system for that. Boats are already a thing.