from my research online and personal experience, YMMV
ok to wash engine bay with regular garden hose with whatever nozzle you have. i like this style in plastic for variability.
i prefer to go a bit sparingly with the water, i dont hose around for 30 minutes...
safer with a cooled off engine, if its steaming when you spray its too hot!!!
initial spray with just water, to remove 75% of dust grime etc
pump spray or aerosol or with a rag, spray/slosh on simple green or similar degreaser/soap
rag wipe down any surfaces that still look grimy and loosen up whatevers left.
final rinse with hose again. can redo spot clean any areas that didnt turn out nice.
like i said i dont spray around like a maniac for 30 minutes, but rinse, wash, and rinse again.
trying not too use tons of water, the adjustability of the nozzle helps.
as its not a pressure washer pushing water in where it doesnt belong is unlikely. and its unlikely to run in just on its own.
follow up with leaf blower to do a quik dry, works like a charm to remove 95% of the water.
after that i will typically go for a long enough drive at high enough speeds to heat up engine and dry out engine bay 100%
i do this about once every year on any of my vehicles.
if it needs a more expert cleaning, or you want it showroom ready, dealer will steam clean it for you for around $100.
and if you think they take great care with the steam cleaner, HAHA. they dont.
they blast that thing all over the place...
my method comes out nice enough that i am satisfied. no problems with any of my cars in the last 20 yrs.
clk 535i 4runner prius xlr lexus cressida(bonus points if you know that one!) and others