My last boat has always been shrinkwrapped during winter lay-up, resulting in no leaks to the cabins or bilges. The masts (ketch rig) were deck stepped, so no issues there.
The boat I had prior to that however, was covered with a tarp in the off-season. That boat always accumulated some water in the bilge over the winter. After one season too many of discovering fuzzy spores in the cabin - come spring-izing, I started using a trick that was the permanent cure to all cabin dampness. I repeated this process on the following boat as well.
Depending upon the size of your boat, gauge these quantities accordingly. For my Nauticat, prior to each winter season, I bought 5 lbs of DampRid, a 5 lb bag of clay cat litter, and three shoebox sized plastic bins from the dollar store. Mix the ingredients 50/50 and fill the bins equally. Disperse them around the cabins with covers off, and your boat's interiors will be dry come springtime.
True Blue . . .
sold the Nauticat