If water gets to any sensitive electronics or high voltage wires you'll be walking.
Depends on a lot of things, vehicle design, engine configuration, splash pan integrity.
In general, if you have spark plug wires that get soaked, they'll short out if they aren't routed firmly and the spark won't get to where it needs to go, electronics don't like being doused by splashing water.
8 cylinder engines have the spark plugs up high, some 4 cylinders have the distributor quite low on the side of the engine and that isn't good. The older a car gets the more spark plug wire clips go missing and the wires flop around.
Not the kind of thing you can predict reliably, but you'll know right away if you went too far.

There is waterproof silicone for ignitions also for electronic connections. On newer trucks the computer is a solid state board that sees no effect by being splashed. Add longer tubes to your differential vents and to your transmission and t case. Your air intake and exhaust should also be above your water expectations. The exhaust is not really needed but if your truck stalls for some reason water will fill your engine.. It all depends on your ride.

One of my biggest regrets has been selling my '81 Bronco. It had no special mods but it was a beast. I once drove through a water hole where I had to roll up my window to keep the water out. I drove in and drove right back out, no problem.
Now I drive a Ranger I bought new in 2007. I've driven it through water deep enough to fill the bed by a couple of inches, with no problems. I don't know about newer trucks but the older Fords seem to do quite well in deep water.

One of the tricks I used to use when doing serious jeeping, working in the bush and even today with ATV, is keeping speed low so you don’t plow the water up higher then it really is.

You look like a real cool dude plowing the water up over the hood, but you may not look to bright when stuck in the middle of it with stalled engine.

Speed and angle of attack Help keep you moving and unstuck.

I had a 1974 K5 Blazer with a intake snorkel, 40 inch Mudders, Rough Country suspension and a Warn Sidewinder. It had some engine work done, but I hadn't waterproofed anything. I almost submerged the cab in a stream while crossing it by accident, but only ended-up needing to hold my breath for a couple of seconds before it came up. I was fortunate (lucky) that it didn't stall because I didn't have an exhaust snorkel. Fortunately back then there wasn't the electronics that they have on them today, so I can only imagine that they'd be a nightmare to waterproof with all of the electronics. My current 4X4 isn't protected because I wouldn't know where to start.