“Mizu kusai” means that food is tasteless. The flavor becomes weak due to high moisture in the food. Literally translated, it means water smell. When you use this word figuratively to describe someone, it means “Don’t act like a stranger/You treat me like a stranger”.
In Japan, “彼は金槌だ” (Kare wa Kanazuchi da, he is a hammer.) means that he can’t swim.
This is a very common phrase. When a person who can’t swim gets an invitation to go to the sea or a swimming pool by friends, he can decline it by saying “I am a hammer.”
As you may know, there are multiple first person pronouns in Japanese. Women use Watashi (私, わたし) for their first person pronoun, but don’t use “Warawa” in general. Originally, “Warawa” was a word that a woman used to humbly identifies herself. In anime or other fiction, it is used by the queen or princess when they address themselves pompously.
“磨穿鉄硯” (Masen Tekken) means to keep a strong will and strive until you achieve your goals. As for the meanings of each character, “磨”(Ma) means to erode, and “穿”(Sen) means to make a hole. “鉄硯”(Tekken) is an inkstone, which is a stationery made of iron. You would put water and grind an inkstick in it produce ink.