Currently, fluids like water are treated as "walls" when building greenhouses, making it impossible to create greenhouse environment for crops in water such as kelp or rice (in farmer's delight).

At present, there are perhaps two possible approaches.

A. Treat the water as analogous to the air.
B. Directly consider crops in water as being in a greenhouse state.

while in any case, conditions underwater are still fundamentally distinct from those in the air.

btw, feel free to share any additional suggestions you may have.

The former one is a more reasonable approach, I suppose