Rory's right - if you keep the boat (BRe or BR20) unballasted on your mooring, the self bailers in the sump will be high enough to drain any rainwater from the cockpit sole by gravity, leaving just a bit in the bottom of the sump. Just make sure that the boat isn't down by the bow, otherwise water will congregate at the wrong end. And you may need to antifoul the bottom of the sump to stop it going green and slimy.
Try opening the sump bailers next time you're leaving the boat to see what happens. Once the sump has stopped filling up, it should always stay at that level with or without rain coming in. To be absolutely certain, you could pour a couple of buckets of water into the boat when you're not aboard, see it fill up the sump and then watch it disappear down the plughole.
I'm not sure what difference ballast would make. A full sump and a bit of water sloshing about on the cockpit sole perhaps? Again, try opening the sump bailers when you leave and see what happens. At worst, you may get wet feet next time you get back in.