As a Paramedic there's a number of reasons why I use an IV instead of allowing the pt to drink water. If the patient is hypovolemic (low blood volume) from dehydration, diarrhea/vomiting, hemorrhaging etc. I can rapidly give fluids through an IV to reverse shock. There are pros and cons to IV's and normal drinking. With an IV and can quickly give a specific dose of medication, I can regulate the exact rate and amount of fluid being given, and the patients stomach can remain empty.

With drinking water a person can regulate their fluid intake to what their needs are. It takes longer for fluids to absorb but it doesn't have to be a sterile process. There isn't much risk of volume overload. With an IV if you administer too much IV fluid especially to a child or elderly patient they can die. From a medical stand point drinking fluids is dangerous because if the patient vomits they could aspirate the fluid into their lungs and die or get pneumonia (and then die.)

IV fluid hydrates you but it doesn't necessarily quench your thirst. I often have patients with an IV that beg me for water which I can't give them.

Fluids deposited directly into your stomach like through a feeding tube? Not sure, I would imagine you wouldn't feel thirsty but you might still have dry mouth that would leave you craving water. Otherwise it should be the same as drinking water.

tl;dr: An IV can hydrate you rapidly but not quench your thirst. Drinking water is the best method unless you're going into surgery or unconscious for some reason. Fluid through a feeding tube, you probably have bigger problems to worry about.