The only bad part about the snorkel and hose is that you may not be able to expel all of the air in the length of the hose, and in essence you would be breathing back in the expelled CO2. It would be like breathing in a bag.
If you do this, you would need to have a backflow preventer, so that when you exhaled, it would push out the purge valve.
Just my opinion. I have actually thought about this same method using a weighted float so you could pull it along as you swim. It would not be good to continually breather the same air over and over.
Good Luck,
Robert