Oh, thats actually true about drowning silently. In media, drowning involves a lot of screaming and splashing. But in reality- especially for kids- what you need to look for is a child being strangely silent while thrashing or struggling to surface. A theory is that the struggle to breath overrides the ability to scream for help.
Ah yes, drown ghosts, everywhere has them (I guess)?
In Vietnam, those fuckers are called "ma da" and they drown you to replace their place so they can pass on to the afterlife. ot sure about Japanese ones though.