Rust on your screws is probably not from water but rather an electrical current flowing between the two dissimilar metals. The rust is from the weaker, sacrificial, metal...in your case steel. If you were put stainless without some type of barrier you will have the aluminum corrode away. Here in Florida we see this very often when folks replace their steel screws with stainless. In fact, my pool cage had some steel fasteners replaced with stainless and the stainless screws dropped right out of the aluminum. Using anti-seize lubricant on any screw will help reduce this corrosion. I use Permatex Anti-Seize Lubricant which should be available in any hardware store. I understand that there are screws which have a coating on them which will insulate them from the other metal.