Asad, water crossing is a very technical event. its not only how deep you can go but how to approach a crossing, whether the water is standing or flowing, if flowing how fast is it, are there rocks or other debris submerged in the flowing water, what kind of surface is beneth the water i.e is it rocky or mud or sand etc, whether you can see the bottom and can judge the depth of it or not...
these are all important questions you have to ask and get answers to before you put your vehicle in water whether equipped with snorkel or not. e.g snorkle will not save you if the water is flowing very fast and carries with it rocks and stuff as is the case in most northern area streams. or if you go to a pond, the water migh just reach your bumper but you'll get bogged down in the silty, muddy bottom.
So to answer your questions, the first part is you need a snorkel ideally to attempt water crossing. if you don't have one then you can safely put your vehicle in the water without causing any technical harm as long as it doesn't reach your air intake, a good rule of thumb is that the water shouldn't reach above your bumper.
secondly for any fast flowing stream or any other type of water try to see how fast its flowing, moving water has surprising force and can easily lift and sweep away vehicles, anything getting up to your doors is going to be exerting tons of pressure so avoid such situations.
Also, make sure you can tell how deep the water is before entering it, as there might be holes or sudden drops beneath the surface, best way is to walk the length of the track underwater.
enterance angle for crossing a fast moving stream is another point, never go directly perpendicular to the fast moving stream, start off a bit upstream and go across at an angle, aim for point higher than where you intend to exit and the flow will drag you to the right spot.
hope this doesn't discourage you from taking on one of the best part of offroading