Hi!! The tale of this spirit differs on who is telling it/ which area it's told, so there are many tales as it was passed on orally. From the description, the tale takes it on from Costa Rica, which may differ from what I will tell you, as I am Mexican, and am going to tell you what I have heard.
Overall, there was this beautiful women, Maria(that's what some people say is her name), who one day fell in love with a man (usually he was a merchant, ranchero, or conquistador) from outside her town. The two eventually married( or not in some tales) and had two children (number of children vary in some places, but there's at least 1 kid). Here's the part where the story changes depending who's saying it:
-One version goes that over time, the man started to fall out of love with Maria, and only starts to dote on their children, but ignores Maria completely
-Another says that the man starts to lose interest in Maria and his family overall and pretty much cheats on her, with some saying that he at one point completely abandons the family to go marry a richer women
Either way, Maria is overcome with a blind fit of rage and ends up drawing her child(ren) because of it, yet, she immediately regrets it and tries to save them, which she fails to do so. Because of this, she ends up drowning herself out of guilt, but because of her actions, she can't enter the afterlife and is forced to roam the earth until she finds her children. Because of this, she usually takes any children she comes across, which is near any body of water.