Introduction
Hello, I'll just like to share a little trick to brighten up your ponds.
Disclaimer: This requires the usage of WorldEdit; it's possible with in-built minecraft console commands but I'm not familiar with it.
      However, the theory still applies, and those without WorldEdit can try doing the same with the client commands!

Tutorial
1. Select a good spot
2. Place kelp up till the desired height (usually I place it up till the water level).
3. Place a block (any block that's not gravity-affected) above the kelp.
4. Select the said block using your wand (//wand).
5. Replace the block with brain coral fan, non-waterlogged (//set minecraft:brain_coral_fan[waterlogged=false])
6. Enjoy!

P.s. Note that the brain coral fan block remains in this state because it's not constantly being updated. If you place blocks directly adjacent to the brain coral fan, the brain coral fan will be updated and will dry out.

Bonus Content
Add more depth to your water bodies by changing up the biome type of a selected area.

1. Select desired area to change the water color.
2. Change the biome type to warm ocean (//setbiome warm_ocean).
3. Reload chunks (either by de-loading chunk and re-loading, or simply relogging into the world).
4. Enjoy!

Edit:
The map in the above image can be found in my previous submission - Floating Isles.
Link: https://www.planetminecraft.com/project/floating-isles-5364186/

Mods:
https://www.curseforge.com/minecraft/mc-mods/worldedit
https://files.minecraftforge.net/net/minecraftforge/forge/