Describe the feature

A frame that acts like mangrove roots in that they are a solid block that can be water-logged.

I made a very poor graphic:

I forgot to mention farming as a possible use case.

Hmm, interesting, I hadn't thought of stuff like cobble generators at all but that definitely sounds interesting. I have some ideas how I could make this, one of which would be to have the player craft the block with a bucket of the fluid instead of needing to manually waterlog the block after placing it. I'll definitely add this to the list of planned block.

I'm not sure about this, especially because the shape of such a block is very subjective. What's the use case and why that specific shape? If you just want a block that looks like a fluid, then you can already do that with any framed block by clicking it with a fluid container holding at least 1000mB of the fluid, such as a bucket.

What's the use case and why that specific shape?

The shape

Rather than the block having the appearance of water, it would be a water source with the appearance of another block.

In my crude graphic, the frame is what would change appearance, and the water would always be visible.

Use Case

It's use case would be for waterlogging; like mangrove roots, you would place the block, then waterlog the block with a bucket.
This functionality is used in cobblestone generators, and for fan washing in the Create mod.

My reasoning

The reason I mention the mangrove roots is to show that this functionality is already in the game. I would like to be able to camouflage a waterlogged block, because mangrove roots are kind of ugly, and not the easiest thing to find; especially in skyblock modpacks.

Farmland irrigation would also be covered by my idea