Doc Vikingo is correct. We have said before and will say again, try any medicine, seasickness or hayfever or whatever, prior to your dive vacation and check yourself for alertness and reaction time.
Some people tolerate a given medicine marvelously and others don't. Know which you are before going under the water.
A second point is to recommend considering cortisone nose sprays such as Nasacort AQ, Flonase, Nasonex, etc. They have no drowsiness side effects.
Hayfever sufferers are likely to have more trouble equalizing in hayfever season than otherwise, since the swollen tissues in the nasopharynx include the eustachian tube opening.
If your non-drowsy antihistamine works well for you, great!
If it doesn't, modern medicine offers a bunch of options, so don't give up.
See you underwater,
John Reinertson