Without this tag, the player just falls straight through it and there is no fog effect. It's a fluid tag, so it lives in data/minecraft/tags/fluids/water.json. Apparently, to get the appropriate fog color you'll have to hard code it.

Thanks for the thorough report!

I actually have another thought about this. Do you think it would be good to make seed oil a virtual fluid, like milk or tea? There'd be no fluid block for it, but it would still have a bucket and work with Create's machinery. I can try and do a PR later. Unfortunately, I can't get the source to compile, so no PR.

Fixed in 20210711a