Claritin (and Clarinex and Allegra) is an antihistamine. It blocks your body's response to some allergens. It might avert sinus blockage before it gets started, but won't do anything to relieve already blocked nasal passageways or sinuses. If it's allergy season, I'd use these way ahead of diving day to minimize potential of sinus problems developing. For relieving congestion, though, you'll need a decongestant like Sudafed. I've been using a time-released variant, available by prescription, called pseudovent, in a pediatric dose. I use the pediatric dose because I don't like what a regular dose does to me (or any of these pseudoefedrine drugs)- make me anxious and cause sleep problems. However, it does the trick of clearing my sinuses for an extended period (about 12 hours) which is long enough to avoid the potential of congesting during a dive. Also makes equalizing easy for me. I use it for a couple of days ahead of dive which really clears out the pipes. Note, though, that if I were -really- congested ahead of diving, I wouldn't go diving since there might be an underlying sinus infection- see your doctor. So far, this routine has worked well for me. You should be careful with these pseudoefedrine-based drugs, though, since some people (like me) can be very sensitive to them. I've also read on this board that for sensitive individuals, they might not mix well with nitrox.
This is not medical advice, just my personal experience.
cheers,
Michael