Hi,
I would like to ask a few questions and give a few suggestions. I hope we can help you with your problem.
If you say it is rain water then I will go that route. I have seen BMWs with bad seals to the computer and a new computer was needed. So lets hope it isn't that. If #1 cylinder isn't firingl, then try switching the coil to another cylinder to see if the coil is bad. It is common for a direct coil (as on the M50 engines) to go bad and take a computer with it. It does happen and there are people who were lucky and did not need a new computer. The other thing I would check is to make sure electrical connections don't have dirt and spray with electrical contact cleaner or WD40 to remove any left over water. A hair dryer can work too for a no-chemical solution. Make sure your hood seals are on correctly and double check they work. If it still rains, from what you tell us, more rain gets in even while you don't use the car. That isn't good because you are not heating the engine up to get rid of any standing water. In the mean time, protect the engine bay.
Questions:
Describe how the car starts.
Describe how it runs on idle, cold, warm.
Describe how the engine behaves when blipping the throttle, when pushing it and letting the revs go up a little.
Have you checked the battery voltage? (low voltage can make computers act weird - and even throw a computer code)
Have you ruled out a bad injector?
What happened with the washer hose?
Any pics of the #1 spark plug compared to another cylinder's plug?
Good Luck.