Couple of thoughts:
The rubber seals on the forward hatch and the pop-top are pretty easy to check; just make sure that they're intact and dogged down hard to compress them.
The seals on the two opening ports in the head are thinner and could be suspect. I'd tighten them as hard as possible and then check integrity by tossing a bucket of water on them (on the outside, of course).
The upper hatch board in my companionway has louvered slots for ventilation. A driving rain at the wrong angle might introduce water.
Many owners of older C-25s have replaced windows, according to the Catalina 25/250 Capri 25 Association web site. Mine have not been an issue.
Through-deck fittings also can be suspect. How's the caulking around your chainplates? They get a fair amount of movement, and the flexing can introduce gaps you might not notice at first. This is important because water intrusion there goes right to the [wooden, rot-able] bulkheads to which the chainplates are bolted.
All that being said, you've got a capable and fun boat. And, with an installed base of more than 7,000 hulls out there, getting parts and advise is going to be a breeze!
Joe - s/v Assignment, Catalina 320
Last edited by jjablonowski; 12-11-2011 at 08:51 AM.