Cover up?? 
I don't cover anything, just run the engine so its warm, do the jetwashing and then drive it after so water doesn't get a chance to soak in anywhere.
Obviously don't blast directly at the ECU and plugs in close proximity, and if you've got a cone filter, don't shoot at that. Also, cleaning the battery is a good idea rather than covering it, crap on it can allow a short between the terminals and drain it slowly, some spray grease on the terminals after is useful though.
Edit - Disclaimer....I've done it hundreds of times without issues, if yours breaks I don't take responsibility.
I don't cover anything, just run the engine so its warm, do the jetwashing and then drive it after so water doesn't get a chance to soak in anywhere.Obviously don't blast directly at the ECU and plugs in close proximity, and if you've got a cone filter, don't shoot at that. Also, cleaning the battery is a good idea rather than covering it, crap on it can allow a short between the terminals and drain it slowly, some spray grease on the terminals after is useful though.
Edit - Disclaimer....I've done it hundreds of times without issues, if yours breaks I don't take responsibility.