How to make aquatic plants

For a plant that grows on top of the water like a lily pad: in the 4th tab of the plant, make sure "Plant Type" is set to "water" and immediately below that the option "Optional list of blocks plant can be placed/grow on" is set to water as well.

If your plant is supposed to grow in/under water: unfortunately, mcreator's plant maker is lacking some features, last I checked this included. Last I tried you have to make a regular block and set it up with procedures/triggers to work correctly. In "Visual" set it to be waterloggable and hide fluid texture when submerged. "Properties" and "Advanced-Properties" adjust things to be more plant-like: hardness, resistance, replaceable, allow entities to walk through it, color on map to water, AI-path node to water, etc. In "Generation" set it to replace water. And this is where it gets kinda dicey.
If you set it to spawn on its own from here, everywhere it spawns it'll be in an air block because it replaces the water. You have to creature a structure where its sitting in water. Then between the structure generation and triggers of the block, set up some procedures so it can only be placed in water and on desired blocks, and so it will only generate in water and on desired blocks. If you make a cluster of these in your structure, make sure to also add a procedure that'll remove it if the correct block type isn't underneath it (this keeps it from stacking on itself or free-standing in the water column near slopes).