I met someone yesterday who didn't like being told a road was closed by a flood... "even with a 4x4?' he asked... (big Mitsubishi). Yes, was the reply... four wheel drive means you can cross slippery surfaces, but doesn't make the car any more waterproof. The water under a low railway bridge was waist high.
To answer the question: several factors here. the air intake to the engine is on a level with the centre of the headlights - so you could say that was the limit. But, there are numbers electronic devices lower down than that, both under the bonnet and inside the car. Get more than a few cm in the footwell and you can kiss goodbye to most of the safety circuits (airbag controls etc)
As well as electrical items, things like the gearbox and rear diff have breathers on them, which need to stay dry. These are not very high off the ground, lower than the engine air intake (when you see Landrovers wading in very deep water, they have (or should have) 'wading plugs' fitted to these vents to keep water out of the gear oil). OK for a brief journey through a deeper puddle but anything which would submerge these (especially when they are warm) is likely to draw water into the system (partly through the cooling effect of the cold water)
Then there's the bits and bobs that go between the inside of the car and under the bonnet - the steering column, brake control and so on. And, at the back there is a large open vent behind the back bumper too...
Fiat's (often inaccurate when it comes to detail) press release says over 700mm. I don't believe that for a moment. Many much larger off roaders have a limit of just 500mm. But this article suggests a more believable 410mm - which i reckon is water that I could, just, walk through with my posh French wellies without it coming over the tops.
https://autoprova.be/2020/02/16/we-...-x-4-your-cute-compact-companion-on-any-road/
I would be very wary about anything deeper than the the top of the sills / bottom of the doors personally... unless other measures have been put in place to keep water out.