I personally don't even wanna try to use those hoses at the car wash or at home. I used a degreaser and got my hands a little dirty but at least I felt better in knowin' that when I got done my car would start.
I think a reguler garden hose is okay to rinse things off, I just wouldn't jet pressure at anything electrical in nature..though engines are pretty waterproof these days, just watch the electronics and an open intake if you have one.
A hose or a pressure sprayer is too much water, unless you have taken the time to cover electrical boxes, ignition components, exposed air filters, etc. I learned this the hard way on my old MR2.
I used a spray bottle of Simple Green and the trusty old garden hose. The engine compartment came nice and clean, but then the car ran like crap. After letting it sit wasting away for six months, I finally took it to a Toyota mechanic who eventually discovered that water had seeped into the Air Flow Meter and kinda "sludged" up and prevented the door inside from moving freely. Then of course I crashed that car the next day after getting it back. Doh!
So, if you use a hose, cover the water-sensitive stuff and/or use a very gentle stream, not a full blast!
After about a year of driving, I cleaned my engine at a car wash with the pressure wash sprayer. Nothing bad happened electically or mechanically (fortunately), but the cleanser they use stripped the wax off my car (don't think it hurt the clear coat though). It took a lot of washing and waxing to get that evened out again. I didn't cover anything and didn't get too close with the sprayer, but I hosed if off good. I'd say a more gentle running water is fine such as your garden hose on mild pressure without any kind of sprayer nozzle. Dish soap is a great degreaser, but try not to get any on your belts unless you have belt lube.
Do you drive your car in the rain? If so, you can use a garden hose under the hood as long as you don't do a few things.
A) fill your intake snorkel full of water (and if you have a volant, cover that.
B)Take out your spark plugs and plugwires and fill in those dirty little holes. (Somebody has actually done that.)
C) Spray directed high pressure water at electrical components.
As long as you don't go extreme on your engine you should be fine with low pressure water, make sure your plugs are tight and that the plug wires are snug on them. This will keep any and all water out. Don't concentrate direct stream on your intake component. As for a cleanere, some of that orange cleaner stuff diluted 1pt to 4pts water will work nice. Spray it all over, then gently rinse away, check the gallery for a pic of my engine, that's how I do it, clean the intake box by hand if you have the volant.
I have washed my engines for years with no trouble. I only use the garden hose and I watch where I spray. I stay away from electrical components and the air filter (if it is exsposed). I use Simple Green. I find it to be the best for the job. But most of all, it take a lot of elbow grease. You have to wipe it down almost everytime you wahs your car if you want it to stay looking good. Also, don't forget the under side of your hood. I hate it when I see a car that has a clean engine and the under side of the hood looks like it has never been touched. I have a Camaro SS and the under side of the hood was as clean as the rest of the car.