OK this maybe one of those real dumb questions but here goes. I've almost completed the DM course and one of the last things I have to finish..is the 800 yrd snorkel swim. I've tried it a couple times and keep having issues with breathing on the snorkel that long. Never have issue with Scuba, but this last time at a little over 500 yrds stared coughing up flem. I'm going to try sudifed a couple hours before my next attempt. Anyone else have this type of issue? If so what worked? This was my 3rd try (yeah I'm going for the record), my instructor is very pacient with me.