It really depends on what type of wheeling you do. Here in Costa Rica, we do A LOT of river fording. Keep in mind trails here are really roads to get you somewhere, and if you need to cross a river, you cross it. Often times, on the way back, the river is deeper than what it was on the whay in, and you are left with two choices: drive through or wait.
Oh, and what was mentioned about the 4-banger wheeling with the big boys is correct; mine is a 4 banger YJ on 33's, and I wheel with V8's on 38's and 42's, and have yet to be unable to go where they go. I just need to be more careful with my lines, and use the winch a bit more often, but I do get through, which is what matters.
Here are some vids of our water crossings
Sorry about this one being 15' , but it is a good one to show the river becoming deeper; about halfway are some still pics of the basecamp, and after that, you will see the river eating up 32" tires on the red Korando and the Gray Toyota Fortuner.
In these vids on the swamp, you will notice a stuck Jeep; he was running 38's TSL's with a I6. It took 3 winches to pull him out...
I dont have a pic of my rig on this obstacle, but I did get through it with few problems (it took a couple of tries, but I made it)...
An another river...
Now, I know THESE rivers did not require a snorkel, but what if the river had grown a bit more? Rather safe than sorry, in my opinion...
Now, like stated above, a snorkel WILL NOT turn the Jeep into a submarine. You can still get water in the ignition (which will stall the engine), and any number of sensors can fail (not excluding the ECU). However, if the engine does stall, all you need to do is dry it up, replace the sensor that died on you, and drive on (I have all the sensors and a spare ECU in the Jeep for this purpose). The snorkel will save you from hydrolocking (and, in all practical sense, KILLING the engine), so it's kid of an insurance. As for performance, you could probably see some ram air effects with the snorkel, but the problem is that you probably need to be around 100mph to notice said effect (in other words, DONT expect any performance gains). I kind of think of it as an insurancy policy; I HOPE I dont need it, but when you do something stupid, you are happy you have it... Oh, and for the trail, I typically turn the head of the snorkel so that it faces back, to avoid having it caught on branches and such (you can see that in the swamp vids).
As for the engine having issues sucking in the air, I have yet to experience any of this, but then again, i wheel mostly close to sea level (not more than 3,000 feet elevation).
For the stuff I drive I really wouldn't be too worried of it being damaged. I have been on the verge of drowning my jeep though. I was sitting in water up to my knees inside the rig. I broke 29 cores in the radiator because a tree (that was on the bottom of the "small" hole) hit it and dented it through. I was about to start sucking in water via my air intake but luckily I stayed like an inch away from that. Got a tug out after the gmc chose the wrong part of the bumper to attach to and pulled it off. Random day, but in the end I drove it out of the trail onto the road and then drove up the AAA flatbed. 
