If we can breathe in fluid in the womb why can't we breathe underwater?

You do not breathe in the womb. everything is supplied via the the umbilical cord.

They inhale small amounts of it. The video said it is fetal respiration. It is just not all the time. “breathing” is the wrong word.

We don’t have gills. Babies have oxygenated blood because their mother’s blood is shared.

In the movie “the Abyss” (1989) is where a rat breathes in a fluid. Is based on reality.

The mom is breathing for the baby. As someone said above, the baby’s blood is oxygenated and given nutrients via the placenta.

The mom and baby don’t share blood though, the placenta maintains a barrier. There is a condition that can be life threatening to the fetus if the mom is Rh negative and the fetus is Rh positive. The mom can send antibodies that can harm the fetus, attacking its blood cells. Rh is the plus or minus after your blood type. For example A+ or A-. First pregnancies usually don’t have much trouble because the mom’s body is just learning how to make the antibodies against the positive blood factor.

The fetus practices breathing movements to strengthen the muscles that will help it breathe once born, but the source of oxygen is really through the umbilical cord.

When the baby is born it quickly switches over to breathing on its own as long as everything goes correctly.

Here is a good article about the history of scientific of trying to come up with the ability to breathe fluids. As @RedDeerGuy1 said, it was demonstrated on the movie The Abyss. The actual diver using it was not real, but the scene of putting the rat into the solution was real. This sort of technology was also used on premature babies for a period and it worked. They all lived and some showed increased lung performance later. A couple died later on, but not from anything to do with the experiment.

That liquid as an oxygenated fluorinated compound, not water, so there was a lot of available oxygen for the rat to “breathe”.