Yes, all antihistamines do make you drowsy, to varying degree, but I don't see that as a major problem. You are unlikely to fall asleep under water, at least in our cold northern Europe waters
I checked with a Swedish diving medicine specialist after writing the above. She told me that they avoid using the older antihistamines these days as they make you generally sluggish. The newer products, which have much less tendency to do this, are considered quite safe. The latest recommendation in the Swedish Air Force and navy is to use desloratidine (Aerius or NeoClarityn, Schering-Plough).