perpetual problem for me - you have to seal your mast. I generally use marine silicone every spring before I step the mast. I seal the comp tip joint and the sheave at the top and also the base. Maybe I'll also get a Hobie Bob to float the top someday. With my leakier mast I drilled a small hole in the bottom and tapped it so I could put in a set screw to seal it. It provides a little drain hole, but if the foam plugs are intact they hold water higher up in the mast.
Here is a pic of the foam plug in an old comp tip:
Here is a pic of me removing the sheave from the top of the mast:
On that mast, when I removed the sheave at the top there was a little foam inside but when I dug that out I found it was actually sealed with an interior cap as hopefully you can see in this image.
I just cut off a few inches of a pool noodle and shoved it in there when I replaced the sheave. In the comp tip I pulled out one of the rubber plugs (flat black rubber on side of mast) and shot in some closed cell foam before I put it back. Not saying you need to remove the comp tip, just for illustration.
I believe there is another foam plug near the base. When I replaced the base on that mast I poked a hole in the foam to allow water to get to my screw hole drain. My logic was that if the mast base was under water I'd already be turtled and the set screw with some teflon tape on it should hold it long enough for me to get back in the corner and pull the mast more horizontal. Maybe that's not so bright but worked for me so far
hope that helps