Meaning: Fish move through the water by moving their fins.
Choice of Words: Fish are aquatic animals that live in water. Swim means to move through water by using fins or other body parts. Through means from one side to the other of.
Alternative Expressions
The fish swim in the water.
Meaning: This means that the fish are staying in one place in the water. They are not moving forward or backward, but they are staying in the same spot.
Choice of Words: In this sentence, the fish are still the subject, and they are still swimming. However, the preposition in tells us that the fish are located in the water, rather than moving through it.
The fish are swimming in the water.
Meaning: This means that the fish are actively swimming in the water. They are moving forward or backward, and they are using their fins to propel themselves.
Choice of Words: This sentence is similar to the previous one, but the addition of the word are emphasizes that the fish are currently swimming. The word in still tells us that the fish are located in the water.