腰 and 腹: waist-area references and the abdomen
Distinguish body locations from idioms containing 腹.
The waist area
腰, read こし, refers to the waist or lower-back area. 腰にタオルを巻きました。 says that someone wrapped a towel around their waist.
腰まで水につかりました。 says that someone was immersed in water up to the waist. まで gives a height boundary, not full-body immersion.
The abdomen
腹, read はら, means belly or abdomen. 腹に手を当てました。 describes placing a hand on the abdomen.
腹を抱えて笑いました。 describes laughing while holding one's belly. Do not turn this laughing scene into a description of pain.
Idiomatic meanings
腹が立ちました。 means that someone became angry. It expresses an emotion, not the abdomen literally standing up.
腹を割って話しました。 means speaking frankly and openly. Learn the expression as a whole rather than reading it as physical injury.
Does 腰まで水につかりました mean that the water reached over the head?
Show the answer and why
No. It limits the immersion to waist height.