What is it like to swim in heavy water, or drink heavy water? First some background.
Water is H2O, two atoms of hydrogen and one atom of oxygen linked together into one molecule. This "linking" is done by electrons; in fact chemistry is done by electrons. The nucleus at the center of an atom influences chemistry hardly at all. Change the nucleus of a hydrogen atom, by adding a neutron for instance, and it still links up with oxygen to make water, because the electron, buzzing around the outer edge of the hydrogen atom, forms the bond.
Most of the hydrogen in the universe has one proton in its nucleus, but approximately one atom in 6,700 has a proton and a neutron. With two particles in the nucleus, this form of hydrogen is called deuterium. Electrons are extremely light, so deuterium is twice as heavy as hydrogen. Water made from deuterium, sometimes designated D2O, has two deuterium atoms and an oxygen atom. It is about 10% heavier than ordinary water, thus "heavy water". In fact ice made from heavy water will sink to the bottom of a glass of ordinary water.
You have just been transported to a cottage next to a lake of heavy water. What is life like?
As mentioned above, the chemistry of water is the same. You can wash your dishes and clothes as usual - even take a shower. Soap up and rinse off, you'll feel nice and clean. Then go swimming in the lake if you wish. It's easy; you can't help but float. Remain motionless, and your head is still above water. But you notice there is no life in the lake, no fish, no plants, not even algae. The water is crystal clear, down to the rocks and sand below. If you had a microscope you might see some bacteria in the water, but that's it.
Go back in the cottage and cook with the heavy water that is piped in from the lake. Make a pot of coffee if you like. The chemistry is the same. Then eat your dinner and drink your coffee. All is well, for a time, but after a few days something goes terribly wrong.
Organic chemistry, the chemistry of life, is subtle and delicate. Everything looks fine in a test tube, but not so in your cells. When 25% of the water in your body is D2O, cell division is inhibited. Life requires cell division, particularly in the bone marrow (producing blood cells), and the intestinal lining where cells turn over rapidly. Death occurs at 50% D2O concentration, though it is a lingering, unpleasant death, as though you were on high doses of chemotherapy.
If the chemistry is the same, what goes wrong? Well the chemistry is not exactly the same. Almost everything you do in a test tube involves ionic and covalent bonds, but life also uses something called a hydrogen bond, which is much weaker than the other two bonds. It joins large molecules together, gently, and it is central to life. Heavy water slows down these reactions just a bit, just enough to interfere with life processes, especially cell division. Subtle is the chemistry of life.
Bacteria grow and multiply using a simpler process, and thus they are able to survive in 100% heavy water.