Generally the decongestants don't cause dehydration.. They work by constricting blood vessels in the body, which in turn shrinks the nasal mucosa and causes the openings in the nasal passages to increase in size to allow more air through... They can have stimulant effects though, which is why they are either banned by WADA in competition, or are on WADA's monitoring list at low concentrations, so be sure to check before a competition if you're subject to drug monitoring. It's usually the antihistamine ingredient in cold and allergy medications that people think dehydrates them. It's not really dehydration though. The older antihistamines have a drying effect on bodily secretions (in addition to blocking the histamine that causes many allergy symptoms), so they can dry out your mouth, nose, and sometimes eyes without actually dehydrating you... They can make it more difficult to urinate too, but they don't really dehydrate you. (It's substances like caffeine, alcohol, and diuretic medications which actually stimulate the kidneys to put out more urine.. Those are the types of substances that have a greater chance of causing dehydration if you don't also drink enough other fluids...)
The newer antihistamines, like loratidine (claritin), fexofenadine (allegra), and certirazine (zyrtec) cause less sedation and have less drying effects than the older antihistamines like diphenhydramine (benadryl), chlorpheniramine (chlortrimeton), triprolidine (actifed), and others.. When it comes to cold and allergy medications, the antihistamine is the ingredient that actually helps block the allergy symptoms and helps dry up a runny nose. A decongestant is sometimes included only to help open up a stuffy nose and to possibly allow the sinuses to drain a little better for people who need that. If your competition might involve drug testing, then it's probably best to take the antihistamine and decongestant as two separate medications. That way you can stop the decongestant a few days before competition and continue taking the antihistamine so it will continue to block the allergy symptoms.