OK, here goes. I unfortunately have a lot of experinece with this.

Get a diesel. No ignition. The electronic modulesare generally mounted up high and phenolic resin encapsulated. Then all you have to worry about is the starter, and there is really nothing to be done about that.

Check where the air intake is mounted. I 'put the fire out' in a new Chevy 6.2 once when I bailed off into a crossing and pushed water into the intake. Luckily, not enough to bend the valves. I moved the intake from in front of the radiator to the top of the inside of the engine compartment. Problem solved.

If a gas motor, there ae only limited things things you can do to waterproof the ignition. Make sure that your plug wires are new. Thsi sounds extreme, but I change mine out once a year. Make sure the plug boots and the distributor boot are tight. The usual weak spot in conventional systems is the distributor cap. You can run a bead of silicon around the bottom of the cap, but be very, very careful not to muck up the inards.

If you da have to ford some water that is up over the hubs, you are going to have to later drain your diffferentials and gearboxes and replace with new lube. If you get corrosion set up in those units, just junk the vehicle. A transfer case costs more than many new cars.

I have driven Range Rovers since 1987. They are built for extreme conditions, and really can take a lot. My 87 went through water that was washing through the floor when both back doors were opened. My 92 has had 3 inches under my feet. The great thing about these guys is that all the carpert in installed with twist locks so you can take it out and dry it. Whatever you do, don't get aftermarket floor lining material that is screwed down under the sill plates. Been there, done that -- water gets under it and stays there.

My best advice is simply don't. Unless you really know your vehicle and it is the right vehicle, you have no idea how little fast water it takes to move a car. Once you have water pushing on the sheetmetal, you can kiss your ass goodbye with all cars and most trucks. The only reason I have gotten away with as much as I have is we drove 3/4 4x4s with lots of really heavy stuff bolted on, like 1/2" steel plate goosneck trailer hitches in beds, and 50 to 100 gallon fuel tanks. 6,000 pounds or so, as rigged.

I have now litigated 16 low water crossing cases. 24 dead, two survivors. One got to watch his freind die when the motorized shoulder belt would not operate when the electrics went. The other survivor was also in a Ford, also with a motorized shoulder belt but was able to wiggle out from under the belt and get out the window. In both cases sun load had welded shut the emergency upper anchor release point. The first was a Fiesta, the second a Probe. the Fiesta ultimately floated and flipped. NOT survivalble. The Probe just went sideways off the road and into a stream channel. That woman was rescued when she washed into a chain link fence 3 miles downstream. She has a hell of a story to tell about having to duck under water to avoid 'low steel' -- the bottoms of the bridges she was swept under. Thank goodness there are few of these cars left on the road.