Clarinase doesn't cause any drowsiness. So that's good. but you shouldn't take too many, one pill for the whole day (within 8 hours). you don't want a reverse block once the drug wears away.
I had sinuses, and still do sometimes.
Once it was so bad and it was during a liveaboard trip to the philipines. I forced myself to dive, and had minor nose bleeds after every dive. Don't do this!!! I could equalise exept that it took ages to equalise properly. so while others are happily diving at >30m, I was still puttering at 10m.

bad news. when i'm finally down there they are already on their way up :-/
when i came back i went to the specialist and turns out i had a sinus infection. I was on antibiotics for a week. it cleared and went diving again after 2 weeks. and it equalised beautifully.
as a precaution, I take 1 clarinase and a nose spray decongestant in the morning. that's it and it seem to work well for the whole day.
If you do have a confirmed sinus problem, go see a specialist. there is a steroid based nasal spray to control sinus problems. I've aso used this before.