I built a 5 by 5 square with the new Dried Kelp block, made the inside hollow and put water in it. The water was flat (this may be important information). I tried putting seagrass in it, then I, seeing that you need to have at least two blocks deep water, dig the bottom blocks out. I then put seagrass on the bottom, remove the four pieces that you place the sand in when crafting TNT, and placed in kelp then flew out. The water had randomly turned invisible.