Try making a "water soil" for planting kelp, seagrass, etc. and it works, but you can't see the water block inside the pot.
I tried with "minecraft:water" and different levels in the properties but none worked.

I don't know if I'm doing something wrong or it just can't be done. (As an alternative use a cauldron with level 3 but of course it is not the same)

Water is not supported because fluids can not be rendered in the pot. Fluid support is a planned feature.

I understand, it's good to hear.

In case someone else wants to add their aquatic plants, I recommend the lapis lazuli block for the display, it is quite similar to water.

Water soil was added in a194d41

Vanilla water crops were added in 899d1a5