Technically; the distance from the floor to the air intake.
Sensibility; the distance from the floor to the bottom of the air intake box or roughly no more than 1/2 the height of the wheels.
If the water level is even deeper than that, maybe approx. up to 3/4 wheel height is the absolute max ( I'd guess on the
Formentor - just because it's raised a little ) you should be crawling forward, driving at a slow consistent speed behind the bow wave.
The key point is even if the water level isn't that high, driving at a speed whereby the water rushes over the top of the bonnet is a hydro-locking death sentence for most cars.