I can't till dirts after I pour water on the dirts. But I can till the dirts first, and pour water on it, they are still farmlands.

The paddy field is included in some agricultural mods, such as Farmer's Delight and Tea the Story. Growing rice in Farmer's Delight doesn't require to till dirts, that is not in line with actual production. Tea the Story has created a unique tilling method, which ignores cultivated area.

So I suggest allowing to till underwater dirts into farmlands. Although vanilla Minecraft won't add more aquatic crops, doing this helps the development of agricultural mods.