Several completely different classes of drugs/things are discussed above. As a lifelong allergy sufferer, I've tried them all. They mostly have different specific uses and effects. Some can be combined.
Let's start with the allergy drugs. These all treat congestion that are a result of allergic rhinitis aka "hay fever". None of these are very effective for congestion caused by a cold or other infection.
The OP asked about Fluticasone Propionate, the active ingredient in Flonase and its generic counterparts. It's a nasal spray that you take daily. It takes several days before it becomes fully effective. It's pretty much without side effects and is inexpensive if you shop around. Costco has the best price per dose I've found. If you have minor allergy problems, it may be all you need. If you have more severe problems, it is likely a good base treatment that will lessen your need to take additional drugs.
Loratadine, generic for Claritin, is a daily pill that is similar in profile to Flonase. It reduces the symptoms of allergic rhinitis and has few and minor side effects. It's also really cheap in generic form. Relief begins within about an hour of taking it so it you don't have to take it every day for it to be effective.
You can combine the two to deal with moderate allergy problems if one of them is insufficient. I used to take Loratadine daily and add Fluticasone in the spring and early fall when pollen counts were higher where I live. I've recently reversed that after reading research that indicates Fluticasone is somewhat more effective than Loratadine as a daily treatment.
Cetirizine, generic for Zyrtec, and Fexofenadine, generic for Allegra, are the other main OTC allergy pills. People react to them differently, but for me they are stronger and have noticeable side effects. For example, I will only take Allegra if I have to go to the house of someone who has a cat. It makes me very drowsy, but allows me to spend a couple of hours with a cat before the allergies become very uncomfortable.
Now let's move on to the stuff that works for any kind of congestion.
For pills, the most effective option is Pseudoephedrine, aka the original Sudafed. This is still available in the US, but you have to ask the pharmacist for it and show ID. This is very effective, but only lasts around 6 hours so you have to be careful about the timing of dives after you take it. Longer term versions are available, but I don't know how well they cut through acute congestion. The main side effect is that it makes you feel jittery.
For very fast, but short term congestion relief, you have Oxymetazoline (Afrin) nasal spray. It can also make you jittery, but the real danger with Afrin is that the effects are so short-lived that the congestion could return during a dive. This could lead to a reverse block where you can't equalize during the ascent. IMO, I would be wary of using this at all if you have a cold, especially for longer dives.
I guess I should mention diphenhydramine, generic for Benadryl. It's useful if you have an allergic rash or reaction to insect bites, although If you can get it a topical formula is generally preferable to a pill. But don't use it as a decongestant. It will help if your congestion is caused by allergies, but there are much better solutions for that. And the side effects of severe drowsiness and muddled thinking are not what you want while diving. Don't mix with alcohol.
Moving beyond the drugs, we have nasal washes. These work surprisingly well for minor congestion, whether from allergies or infection, or as an adjunct to one or more of the above. Most people are aware of neti pots. But they are not easy to use correctly and most people who try them give up before figuring it out. A much easier alternative is the Neilmed Sinus Rinse. I'll put a link to their site, but it's available on Amazon and most drugstores.
Neilmed Sinus Rinse
shop.neilmed.com
That's the major OTC options. Again you can combine these if just one isn't enough.