That really depends on the type of fish and where in the ocean it is living. For many fish, the biggest challenge is trying not to be eaten by other animals in the ocean. Fish that are small prey face the constant challenge of survival every day and many will meet their end in the stomach of a larger fish or mammal. Though like humans, fish can also get many diseases and parasites that can wipe out large numbers of fish at once. This is more of a problem for fish grown commercially than fish in the wild. Finally, one of the greatest challenges that fish face is human interference. Human activities can pollute the water and raise its temperature which lowers the ability of fish to survive. Also, fishing for a particular fish irresponsibly can lead to a particular species decreasing in number.

Ocean fish have less salt in their blood than the ocean itself, which means that they have to constantly take in water chemically and excrete salt in order to survive.

All kinds of fish face different challenges, and each are adapted to these challenges uniquely. If I had to choose one particular challenge, I would say breeding. Many salt water fish have to swim very far upstream, even into fresh water, to lay eggs. Millions of fish die doing this every year, but still they spawn.

I think another cool challenge is how arctic fish adapt to cold. The water in the arctic regions is very cold, but fish still live there. They adapt their cells at a molecule scale by removing cholesterol molecules from their cell membranes, making the membrane more fluid, so it doesn't freeze as easily at cold temperatures.

This is pretty cool.