You can't expect to keep water from entering the boat from inside the mast. there are simply too many opportunities for water to get inside. What you can expect is to stop water from entering the boat around the mast. A properly installed spartite system will stop virtually all water from entering around the mast and a boot will stop the rest. A little fresh water in the bilge once in a while won't hurt a thing if you pump it out often.
"Keep yer limber holes open"