+1 to what KK said, triple check.
It's not so much the rainfall as most if not all of it will shed out of the deck scupper vs. drain into the bilge. It's moreso that you're mooring on water and the boat naturally can take on water through the carbon seals or unknown leakage elsewhere.
FWIW, I have moored in slip all summer with no issues, but it's partly because I'm anal about checking the bilge, elect, and pump functions.