Some are filled with cement. Some are filled with river gravel, small to medium sized stones. A few are allegedly hollow, I have not encountered one of those.
Typical water intrusion points are the aft shaft coupling (most common), the stuffing box fitting, and shaft log crevice corrosion. Holes in the “fake bilge” as another poster said.
How much is it dripping? Where is it dripping from? Do you live up North, or down South?
What size m/t? Single or double screw?