I fall asleep in the bath tub a lot?!

In theory it isnt that dangerous. If you were to slip under the water you would get a huge nosefull of water and wake up completely freaked out. However if you sleep deeply or take any medication you may not wake up and you would drown.

Also it depends if you do it on purpose or not. If its on accident you may have blood pressure issues. Personally I wouldnt take a chance on it, although I have done it in the past.

Yea, you should go talk with a doctor because you may have Narcolepsy.

http://www.ninds.nih.gov/disorders/narcolepsy/narcolepsy.htm