A: Ah, Yamada-kun, kekkon shita soo da ne. Mizu-kusai na. Hitokoto mo itte kurenai n' da kara.
B: lya, shimpai kakeru to warui to omotta n' da.
A: Yamada, you got married I hear. You might have let me know, I thought. Very unfriendly.
B: Sorry, but Ijust didn't want to bother you, that's all.
* * *
To dilute beer with water will spoil it and provoke disgusted cries of "mizu-kusai." A close friend's behavior is also "mizu-kusai" when he behaves toward you with unexpected reserve or is less frank or open than usual.
Thus, someone who enters hospital but decides not to inform his friends for fear of bothering them is still acting in a way that is "mizu-kusai." His friends have a right to know and are bound to feel slighted and assume that he did not think them important enough to be told.