Well, there are several different ways. Fish that live in clear tropical waters can probably do some short-range navigation by sight and Predators from the deep ocean like Giant Squid have been found to have incredibly large and complex eyes designed to see as much as possible with the little light available, meaning that they may hunt prey using their eyes. This is all short-range stuff though. The really amazing things happen when you start to think over longer distances.
Sharks, have an incredible sense called ‘electroreception’ where they pick up on the incredibly small electric fields generated by their prey. They use this mostly when they’re hunting prey though and use an incredible sense of smell to find their way to their prey, often from many, many miles away. Other fish that live in murky water life Catfish use electroreception through their ‘whiskers’ (or Barbels, as they’re properly known!) to find their way in the gloom.
Echolocation is used by toothed whales (including sperm whales, porpoises and dolphins) They use very high frequency sounds (that we can’t hear) and ‘bounce’ them off nearby objects and interpret the returning sounds (the echo) to build a picture of the world around them. Baleen whales, such as Blue Whales can call to each other using Whale songs across thousands of miles and this may be used in order for Whales to navigate towards one another at certain times of year.
There are many other ways that marine animals navigate, some of which we don’t fully understand. What we do know is that may creatures have developed incredible capacity for finding their way in a world where individuals of the same species can often be separated by incredible distances!
While Sean covered most of it – he left out by scent and taste! They can play a huge role (in sharks, for example) in tracking prey! Try this experiment at school. With your teacher’s permission, pour a trail of sugar on the floor. Now – walk over to the thin end, close your eyes, and start licking the floor! You’ll be able to follow the trail of sugar back to the other end! It’s a little bit more complicated than this in the water, but it’s the same basic idea.