tape a bag around the distributor and alternator (if you can get in there), cover the filter if exposed in the engine bay.

I recommend GUNK engine bay degreaser. It sticks to surfaces and begins to foam up, this allows you to use a low pressure water hose to easily rinse it off so water won't get into areas like under the spark plug boots, fuse boxes, etc. Plus, it conditions hoses at the same time.