Ok, let me ask same question from you. What do you find so sensitive with engine which can be affected by water.
I like to keep my engine clean, so I always wash it with water and surf. Sensitive Areas are Ignition coils and water should not go into sparkplug holes. Put a plastic bag on it and you are good to go.
All sensors are well protected inside its casings, hope you don?t have a loose connection otherwise it does not matter.
My engine did not had a plastic cover on so I bought it from junkyard.
P.S: 10 years means, all the cars I had during this time.