Which fish cannot swim in sea?

This is a bit of a trick question! There's no fish that *cannot* swim in the sea.

Here's why:

* Fish are adapted to water: Their bodies are designed for swimming. They have fins for movement, gills for breathing underwater, and streamlined shapes to reduce resistance.

* Saltwater vs. Freshwater: Some fish are specifically adapted to saltwater (marine) environments, while others are adapted to freshwater. A fish that lives in a freshwater lake or river might not thrive in the ocean, but it could still swim in it.

So, while some fish might not be as comfortable or able to survive long-term in the ocean, all fish are capable of swimming in water.