My real question here is how long would the person be in one of these tubes? Problems that astronauts will face on long journeys into zero-G, like muscular/skeletal degeneration, could be a major concern. Not to mention the drop in cardiovascular endurance from being completely sedentary. So if you were discussing super soldiers or anything similar (since this is the most common use for human suspension tubes that I've seen in sci-fi) you'd probably just be better off breeding the stupidest, shortest, and stockiest of the population until you have dorfs.
Electrical stimulation of limbs and muscles and such. That might solve some of you problems at least. Sure, it would look rather creepy with all those electrodes and such, and peoples spasming in the tanks, but that's better then dropping dead when you get out of the tube.
Degeneration and such strongly depends on which type of tube is used, and most importantly, if the human body is put in some sort of cryo/hibernation. The hibernation system would lower degeneration seriously, as the human biorythm is seriously slowed. The other system would most likely not be used from long time, and therefore not give much problems.
As for dehydration, it depends on the way the system works and such.
General rule of Thumb
If the system is designed a system to provide water, you would need it.
If it is not, you probably don't. The water will most likely provided with the food.
However, for a long time hibernation like system, human digestion will most likely be disabled, and therefore food, oxygen and water will most likely be delivered directly into the blood stream.