Drowning is used as a symbol of losing the power to live. So of course you would be gasping for breath. Try to think of what you are living through right now, and where you feel so threatened. Perhaps something happened to you in the past, and you have unresolved feelings or fears about it? Your dreams maybe trying to alert you to something dangerous, that has skipped by your conscious mind, but your subconscious mind has picked it up. Instead of going back to analyse the dream, look more into your everyday Life and ask yourself, where am I so frightened that I am quite breathless? Something eventually will pop into your mind.