Just as the physical properties of ocean water and lake water differ from one another in some ways but relatively similar in others, that is also the case with much of the biological life in the respective environments. On one hand, many freshwater animals (such as beaver, muskrat, mink, and otter) typically live in lakes but also spend a significant amount of time on land. Meanwhile, ocean seals and penguins spend much more time in the water by comparison. Likewise, the organisms living in the ocean tend to grow much larger than their freshwater counterparts.

On the other hand, some of the freshwater dolphin, sharks, and other lake and river-dwelling species also grow quite large. Furthermore, there is also reliable evidence that some of the largest saltwater predators such as bull...

Other similarities between saltwater and freshwater species are quite evident even from a superficial inspection and comparison of saltwater sharks, oceanic dolphins, and deep sea-dwelling lobsters (for just a few examples) with freshwater catfish, river dolphin, and freshwater crawfish species.
Generally, the specific composition of saltwater and freshwater is different but they are also quite similar. The same is true of saltwater ecosystem species and freshwater species. Ultimately, their differences are attributable to the different environments in which they evolved over hundreds of millions of years; their similarities are likely the result of having common ancestors at much more distant points of their respective evolution.