The water level should not be a problem if you design the tank/filtration right. What I mean by this is #1 you shouldn't have your overflows installed so high in the tank, that the operational water level is near the full capacity of the tank. #2 If you have a large enough sump, and your system is designed properly, it should easily handle the water displaced by your body.
I would say that your biggest concern with going up to 6' deep is the water pressure. As you increase the depth of the tank, the pressure on the walls increases by huge amounts. The result of this pressure and stress is the tendency to bow, or even worse, split the seams. I don't know if you've seen the pics of my tank and how I built it, but I can honestly say there is no way I would trust my design at 6 foot.
Once you settle on some dimensions, post them here and I'm sure you'll get plenty of good advice from everyone who has done this.