Seems the only way you could have salt under the engine cowling is to have salt water getting in there, so a little directed fresh water and soap could hardly hurt. You just don't want to hit it with a pressure spray, I would think. There are some sprays available that say they dissolve salt and you can wipe them away, but I haven't tried them.
It does not matter what method is use if it remove the salt. Putting something over the engine and electronics is common sense you do not want soapy salt water all over he engine. I have to agree with CAP’n Ron if you have salt water getting on the engine, a little soap and water is not going to hurt anything. I would pick a warm day and dry everything out using a big fan.