DivingDoc,
Blood pressure is always higher than ambient pressure -- that's how circulation happens.
In topside IV systems, the bag is located above the patient, and gravity generates the pressure differential to overcome blood pressure and push fluid into the bloodstream.
Underwater, gravity still works the same, and the increased ambient water pressure would have no effect on the feed -- but buoyancy forces would. In fact, buoyancy forces control the feed of topside IV's, too -- the volume of air displaced by the IV fluid weighs much less than the fluid itself, so the fluid flows downward -- just like a rock sinking in a lake. The IV fluid will not flow underwater, though, because the fluid experiences a buoyancy force that is approximately the same as its weight.
Consider the following thought experiment:
Take a tube -- say, an IV tube -- and attach two flexible plastic bags to either end. Consider the lower bag to be your body, and the upper bag to be the IV bag. Fill the tube and bags with water, and color the upper bag's fluid if you'd like. Put the whole mess underwater, and look what happens: nothing (other than your coloring diffusing about). The fluid in both bags is neutrally buoyant (i.e. "is weightless") underwater, and without the buoyancy force, the fluid will not flow.
In fact, because your body mechanically produces a higher-than-ambient blood pressure, a real underwater IV would just result in blood flowing easily into the IV bag. The only way to overcome this would be to use a pump that creates an IV fluid pressure higher than blood pressure.
- Warren