Better River is an add-on that will transform river biome into warm ocean. This add-on will add all of the warm ocean component to river biome. So you will see some coral, tropical fish, dolphin (basically everything that you see in warm ocean) in river.
Below are screenshots of better river addon
Make sure to turn on all of the experimental gameplay
Also, it is recommended to start a new world with this add-on, or the
river biome may looks like this if you don't.
If you make review, gameplay, or showcase about this add-on, don't forget to put the original link (mcpedl page).