I've seen a rash of used comptips on the market these days in questionable condition. Where are they all coming from? I saw 3 at the shop last week, which is unheard of. Guys scored them off of ebay, and they were all advertised as brand new. One, the mast head was ripped off, one had a bunch of cracks, and one had water in it. Weird.
Anyway, water can creep up cracks between the foam and the comptip wall and get stuck in voids in the foam.
I have no idea how to properly fix this, but I'll tell you what I did.
One time I dug all of the foam out of a comptip and had Hobie refoam it. I kind of did it myself using their tools. Another one I took a piece of rebar and drove it into the comptip until I got to the void, drained the water, dried it out in the sun for a few days, then sealed up the ends.
Wait until someone else posts some better ideas, that's just what I did.