Certain plants can be placed underwater, such as Leaves, Vines, Lily Pads, and Dead Bushes, but others cannot. Be consistent, please. This is ridiculous. 

Certain plants include:

   -Saplings of all types 

   -Cactus

   -Sugar Cane

   -Farmland/Crops/Stems

   -Flowers/Double Flowers

   -Nether Wart

   -Chorus Plant

   -Mushrooms

   -Fern/Large Fern

   -Grass/Tall Grass

   -Bamboo

   -Cocoa Beans