It is highly discouraged that you drink ocean water! Because there can be a lot of salt content in ocean water, drinking it causes cells in your body to lose water (the opposite of what drinking water is supposed to do!). This will lead to muscle cramps, dry mouth, thirst, and dehydration. Saltwater fish, on the other hand, have adapted to keep the water level in their cells level. Because they live in such salty conditions, fish constantly lose water and they must drink water through their mouths and filter out the salt in their gills. Freshwater fish do not face this problem and do not have to actively drink water. Gills on a fish also serve like lungs to absorb oxygen from the water and release carbon dioxide back into the water. Fish and other marine species can be threatened by bad waters. For instance, an oil spill in the ocean means that fish will be covered in and ingesting crude oil from the polluted water. This can restrict their breathing and poison them in the long run.

Ocean water is very salty, saltier than our blood. You can drink a little bit because you can drink a little salt water; all animals can, in fact. But drink too much, and you will die, because too much salt is poisonous (it sucks all of the water out of you causing you to die of basically thirst).

Most animals that live in the ocean have blood that is about as salty as the ocean itself, and as a result they can drink/breathe the water, because salt only sucks water out if that water is less salty than itself. Animals that live in fresh water suffer the same way we do in ocean water; too much of the water gets sucked out of them and they die. Again, it depends on the amount of ocean water.

Actually, people can't drink ocean water directly. If you ever tried it, it's not a very pleasant experience because it is SO salty. In fact, if you drink a lot of ocean water, the large amount of salt will actually "dehydrate" you. Dehydrate means it will absorb the water in your body and make you MORE thirsty. For people to drink ocean water, it must first be "desalinated," which means that the salt is removed from it at a water treatment facility.

On the other hand, salt-water fish and other animals including mammals can usually drink ocean water, but their bodies have evolved to handle the large amounts of salt. These animals are able to get rid of the extra salt through their urine or through their gills. People are not adapted for this, so drinking too much salt water could actually be dangerous to a person's health.

You asked how fish breathe ocean water. Fish use an organ called their gills to get oxygen from the water. Fish do not have lungs like people and ocean mammals do. Dolphins and whales are ocean mammals (not fish), and they breathe regular air through their blowholes, which goes to their lungs. They hold their breath when they are underwater just like we do, but they can hold theirs for much longer.

Lastly, you asked about bad ocean waters. I think you mean ocean waters that are polluted with chemicals and materials that are not naturally found in the ocean. This water can be dangerous to both humans and animals for all purposes including swimming, bathing or drinking. Pollution of our oceans is a serious problem and requires agreements between all countries of the world to create laws that keep the ocean clean.