I guess anything is possible, but highly improbably for water to get stuck within the sinuses. See: [link to www.aaaai.org]

There's a very good illustration there. Generally the principle of irrigating the sinuses is an old one. I've found many old doctors who have recommended it with saline as a healthy way to remove material when you have a stuffy head. The sinus pressure usually is pushing on cranial nerve five, the Trigeminal, or cranial nerve seven, the facial, and hence that pressure irritates the nerve causing pain. This is why irrigation helps.

Young doctors might not know about it, or be leery because anything that they recommend, they can get sued for.