The active ingredient in Benadryl is/was the same active ingrediant that was used in the original formulation of Dramamine.... it makes you very sleepy and it is an effective anti-nausea med, BUT be very careful when dosing children.
Caution should be exercised when giving it to children. You will be hard pressed to find any pediatrician who will advocate giving benadyl to a child under 2 years old, it is generally discouraged. Children have died from Benadryl overdoses, so be certain to contact your pediatrician for the proper dosage according to your child's weight and not their age. Be certain to bring a proper medicine spoon/syringe to ensure accurate dosage as well. Also be aware because the drug will probably make your child drowsy and the only activity they will probably want is sleep.
On a brighter note, we have cruised many times on pretty rough seas with our children, and for some reason, small children appear to adjust to the rough seas, a lot better than adults. There have been times I have felt mildly queasy and the kids were oblivious to it.