Riser in Barrel Underwater for Hot Tub?

I have a 7ft diameter steel tub and am thinking of cutting a hole in the bottom and welding a barrel upside-down. The fire box would be outside the tub and the riser would come up inside the barrel. The stratification chamber would be under the whole tup. I'm wondering if the cool down effect of the water on the barrel will slow down the draft too much and if I can compensate by just having a taller riser.

I have considered doing a thermosyphon but would like maximum heat transfer for quickest heat up.

The the other thing i'm wondering (and i know should be on a different thread), is if anyone has experience running hot tubs without any treatment at all since waste water will go onto the land. I'd like to use pond water and just change it out often. Maybe running it through a sand filter, uv filter and occasionally ozonator. Obviously depends on temerature and use etc. but does anyone have experience doing this and suggestions on how often water would need to be changed?

Aaron, I can only speak to the pond water idea.  Honestly, even when using fresh well water, it becomes cloudy in a short time.  I reckon that is due to skin cells shedding and pollen/ leaf introduction.  
Best to start with the clearest water available,  

Warming up pond water might lead to ghastly pathogen proliferations.

Douglas Campbell wrote:Warming up pond water might lead to ghastly pathogen proliferations.

I suppose this depends on how hot the water gets. 160F will kill a lot of things. And 212F will kill just about everything. But you still might want some kind of filter, because killing any pathogens and bacteria won't make the water clear.

Hot tubs do not get nearly hot enough; most run below 104F.
Fill it, heat it, use it, and drain it, all over one weekend.
There used to be a "snorkel stove" wood burner sold specifically for hot tubs.
Will a rocket stove work?
Yes, probably, but your stratification chamber would need to be small, or it will stall on you.
How long would it take??? I'm guessing quite a while (many hours)
Perhaps simply putting your tank on a stand and building a fire underneath might be quicker.