Secretfangirl

Well ...i can give you my theory :)
1. Sweet water=tears - as you can see in the newest chapter, Norihiko likes when Ikurou and his mother are sad, depressed or angry, that's why their tears tastes sweet to him 'cause he is a motherfuckin' sadist
2. Mantis - at first i thought it was a metaphor of Razou, but now i think it's Norihiko's metaphor. I guess this metaphor is all about giving pleasure but taking bigger price than it is worth of - like giving phisical pleasure and an illusion of support but taking their sanity away in return....
But it is just my theory :)

Here is mine:
1. Water = Feelings of nirvana, love, lust, jealousy, admiration, etc. Strong emotions. Sexuality. I think so because it says things like that in the pages 4, 9, 10. Norihiko says he's never experienced these "ordinary reactions" and has never known "the taste of water".
2. Mantis-Horseworm = A parasitic relationship. Sick, twisted. Harmful. Norihiko tastes water (through his experience with madam) and gets infected with this sickness (the Horsewom). He, then, starts infecting others (Ikurou). Sorry, I can't get into much detail. (-_-)" Add Hachi's mantis explanation to mine, lol.

hah I like your theory nice

i think in the relationships--- ranzou-father, ranzou is the mantis (eating and being eaten kinda thing) and in ikurou-norihiko, ikurou is the mantis in the harmful/parasitic/toxic relationship with norihiko (the horsehair worm).
wAter could refer to what naomi said.