Generally speaking modern engine bays are pretty water tight already and I know some detailers who will just go straight in with a pressure washer and get to cleaning. If that makes you uncomfortable then what I do is just use some foil and wrap it around the connectors I see and put a bag over the alternator for good measure. If you are going to use a pressure washer for rinsing use a wide setting and don't force water anywhere, if its a garden hose just keep the water moving and don't saturate one spot for more than a few seconds. I have done my engine bay more times than I can count right now and the last few times I just used a pressure washer without covering anything after a degreasing and never had any issues. Just make sure all the caps and connectors are tight and you should be fine. Many people get paranoid detailing their engine bay and its understandable but once you do it once you realize its not that big of a deal.

Just for good measure on your car though I would bag the intake filter for peace of mind.

 

I've been washing mine since new. I don't cover anything, spray on Simple Green, use a paint brush to scrub any areas that need extra cleaning, hose off and blow dry with a electric leaf blower. Only problems have been the knock sensors under the intake, they have plastic caps that seal them off but water may still get in and short them out. So try to avoid getting any water in between the front of the intake and valley cover, maybe stuff a rag or something in there.

 

Be careful spraying around where the intake meets the cylinder head. Lot of guys who've hydrolocked their motors due to water getting in between old seals.

 

It's probably a good idea to replace the intake manifold gaskets as old as our cars are now. I pulled my LS6 intake that had been on the car for 15 years and gaskets were in sad shape.

I've used engine degreaser and a low pressure garden hose sprayer a couple of times with no issues. Stayed away from the alternator, battery and intake manifold seals.

 

I wouldnt use a pressure washer. I have used a garden hose and some foaming type citrus degreaser ( not the oily gunk stuff ) with relative success. Relative to quickness. That said nothing beats a can of wax and silicone remover, a bag of fresh terry rags, some tooth brushes and larger brushes and an afternoon of effort. Sometimes its easiest to pop the intake off, clean the heads, fill the ports with rags AND tape off the port openings and then you can reach a lot of the other bits easier, clean the coil packs when they are off, paint bits and pieces that need it. For black roll bar black epoxy works good ( its not epoxy it just says that on the can ) its all time + effort and only you can decide how far you want to go. There is no quicky fix that will have it looking new. Once its spotless clean? Regular washes will easily keep it that way