I beg to differ slightly. While carbon frames themselves are virtually immune to corrosion or water damage, the parts threaded into them aren't, especially alloy BB cups. As HillRider correctly states, water will drain to the BB shell, which is sort of the bilge of bike frames. What happens next varies bike to bike, and affects frames of all materials.

If there's a weep hole the BB will drain and you'll have little or no problems, if not water will work into the BB threads and cause nightmarish corrosion problems, in extreme cases fusing the cups into the frame.

Regardless of frame material, don't worry about sealing, focus instead on providing good drainage, and use best practices including anti-seize when installing bottom brackets.