Can sinus congestion pills be taken while diving?

Your mask is fogging so you let in water? That's pretty easy to solve. Scrub the inside of the glass with toothpaste, or if you feel more adventurous, heat the glass with a lighter flame (very carefully) to get rid of the coating that's causing the fogging. (if this is the case)
But yes, I take decongestants now when I'm free diving. About a year ago I started having my eustachian tube in my right ear clog up after 20 or so dives. It gets hard to equalize going down and coming up free diving. I've since made sure that I drink a LOT of water all day long to stay fully hydrated, and have found that taking a decongestant helps. Sometimes I take a second dose if I'm in the water for more than 4 hours.

decongestants and mucinex are helpful - take a decongestant without pseudoephedrine so you won't increase your heat rate and definitely drink lots of water and avoid coffee, energy drinks, and alcohol .
You also need to solve that mask problem -that will help. Are you putting the edge of the hood over the skirt of the mask ( for a good seal) ? Do you think you need a better fitting mask?

Quote from O2 deprived

decongestants and mucinex are helpful - take a decongestant without pseudoephedrine so you won't increase your heat rate and definitely drink lots of water and avoid coffee, energy drinks, and alcohol .
You also need to solve that mask problem -that will help. Are you putting the edge of the hood over the skirt of the mask ( for a good seal) ? Do you think you need a better fitting mask?

Thaat is funny.. I take a Claritin d 24 when I wake up in the morning followed by a giant cup of Cafe Bustello. If I don't do this my ears lock up and I can not dive.

The decongestants can solve the symptoms, but you might want to address the core problem. Keeping your ears warm helps a lot of people. The hood usually does it, but based on the severity of the problem, you might want to try Doc's Proplugs.

I'm not sure why water getting in your mask makes your ears hard to clear. It also sounds like you have suffered an Ear Squeeze, which is why you can't hear. Its caused by not equalizing enough and stretching the eardrum. I have to equalize every few feet so that I don't hurt my ears. Its more important early on, because the pressure change is so great.

Avoid Dairy to reduce mucous production.

Learn the Frenzel technique for equalizing as you can do it faster and easier, if you haven't already. www.spearo.republika.pl/frenzel.doc It's complicated, but I learned it a couple years ago and its what I do if I can't hands free clear (which is a work in progress, but was able to do it all day yesterday).

the pro plugs might help-it is harder to equalize in cold water that we have here in Cali. You are getting a squeeze ( look up barotrauma). As JImCuda says; you need to equalize early and often. Once you miss an equalization you cannot catch up. Equalize on the surface before you descend then every few feet. If you are getting stuck at 30ft or so -it may be because of the thermacline (colder water) and/ or your head position. Keep your chin tucked ,stay relaxed. If that doesn't work; turn toward the surface ,equalize, then continue on.Practice equalizing at home, dry--many times a day. This will train your ears and make the eustachian tubes more flexible. Take a couple advil before you dive to reduce inflammation.
Frenzel is also good to learn- it has to do with closing off the throat and using your tongue like a piston to force air into the tubes. Look up 'equalizing" on DeeperBlue. and read posts by Eric Fattah.here is Eric's famous document http://liquivision.com/equalizing.php
I know this all sounds very complicated but with practice it will happen naturally. Also, get a mask that doesn't leak-- that is annoying when diving and inhibits relaxation.

I would ask that, to a Doc, while we might have had experimented here with a
few things, I would not dare to recomend anything.

What mask are you using, it would seem that controling the water
inside the mask would be the way to go.

Quote from Roberto Reyes

I would ask that, to a Doc, while we might have had experimented here with a
few things, I would not dare to recomend anything.

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That's the best advice.