Follow up as there's been no notice that this was solved:

Per https://minecraft.fandom.com/wiki/Resource_pack#Animation
"If no .mcmeta file exists for a texture with unequal dimensions, the texture appears as a purple and black checkerboard. "

As atexture intended for an animation would be of unequal dimensions, this seems likely.
Note that the .mcmeta file name mus correspond exactly to the filename of the file it is intended to control (including the .png) for it to be recognized;
e.g. nether_portal.png.mcmeta NOT nether_portal.mcmeta

Either missing .mcmeta files as mentioned by others or too low pack format - from personal experience huge pack format number difference will result in simillar water behaviour.

You can also check the logs (.minecraft/logs/) to see if game gives any error/warn messages in relation to this issue.

Knowing what version and upstae of the game you're working with could be vital.

I've not tried editing water, but — from the structure of the assets folders — it should be comparatively straightforward.
The ftexture being animated may be a complication so you will need to ensure the aplicable mcmeta files are also present (e.g. water_still.png.mcmeta)


Will be following to see if there's something I've missed….