I had just created a new world in the new 1.13 version and was planting Sugar Cane around the lakes near the village I stumbled upon. When I got bumped by a villager and I was able to plant the Sugar Cane in the water on a full source block (meaning it wasn't flowing). I thought it was just going to pop and float to the surface but nope it just stayed there. I was guess it might of been an error but I'm able to place them in random odd places in the water. Some blocks won't allow me to place the sugar cane where as other blocks allow me. They grow like regular sugar cane.