Regarding the umbrella question, you need to provide all your own rain gear. Trust me that if NCL had extra umbrellas they would charge you a one-day rental that cost more than the umbrella
You must pack for all possible conditions. We recommend looking at the long range weather forecasts that you can get on Accuweather (they go out to 15 days) which, while not perferct will give you a good idea of the weather trends and temperatures. We always pack a couple of small Tote umbrellas and often take rain ponchos on our trip.