Wow. Sounds like you have a leak somewhere. Water can get in to your sinuses either through your nasal passages, by inhaleing through your mouth, or in very rare and unlikely cases through a ruptured ear drum where it travels through the eustacian tube to the throat and then is inhaled (as I said, very unlikely).
I would check for leaks in your mask or possibaly in your regulators second stage (mouth piece or diaphram)