Im really impressed that you thought of gills! A study last year showed that gills do play a role in water balance for bull sharks. Another part of the answer seems to be in their kidneys.

Heres a little background. Water balance is critical for all living things. Animals that live in salt water have to make sure they have enough fresh water in their tissues. To understand how this works, lets talk about a process called osmosis. Water is attracted to things like salts, proteins, and such. If we have a membrane that allows water (but not the dissolved salts and stuff) to pass through, water will tend to go to the side where the salts are.

Cell membranes leak water, so if you put a cell into salty water, the fresh water in the cell will leave. The cell gets dehydrated and cant survive. Salt water fish keep their outside somewhat waterproof, but salt comes in through their gills and when they eat. So they have to do other things to protect themselves. First, they have kidneys that are good at taking the salt out of the blood and putting it into urine. The kidneys also hold onto urea (the waste product made from breaking down proteins). They also have rectal glands that can remove salt from the body.

Keeping urea, proteins, and other things dissolved in their blood attracts water in from the outside. This makes the water part of the sea water come in, while leaving the salt behind. Sharks keep just enough stuff dissolved in their body to equal out the concentration of the sea water.

All of this would be a problem for a shark that you suddenly dropped into freshwater. Fresh water would flow into the shark, attracted by the urea, salts, proteins, and such. This would kill the shark due to cell damage and the fact that some of the chemicals in the sharks body would be diluted too much to do their jobs. Also, they would lose the salt that is essential to making their bodies work.

If a bull shark slowly moves from the ocean into less salty water, its body can switch over to deal with fresh water. The kidneys get rid of the urea, but hold onto the salt. They produce large amounts of very watery urine to get rid of excess water.

Back to your question about the gills, researchers found that bull shark gills may use salt pumps to move salts into the body in freshwater and out of the body in salt water. These pumps are so small that you could not even see them with our most powerful microscopes, but there are so many of them that by working together they can move a lot of salt.

Do you think the sharks use their rectal glands in freshwater? Why might it be an advantage for bull sharks to be able to swim up rivers? An Australian study showed that a lot of the bull sharks in the rivers were younger ones. Why do you think that is?

If these sorts of questions interest you, you may want to think about a career in physiology or marine biology. You are already thinking like a scientist.