place name
渤 is a rare kanji used primarily in the Chinese place name 渤海 (Bohai Sea). Its standalone meaning is tied to that geographical reference and has no clear independent sense beyond a place name.
渤 combines 氵 (water) with 勃, which likely contributes the sound. The character was created to name the Bohai Sea, a body of water.
The water radical 氵 beside 勃 suggests a body of water. Think of the Bohai Sea: 渤 is the water (氵) of Bohai (勃).
For ボツ, imagine a boat (ボート) lost in the Bohai Sea: boat -> ボツ, and the water radical 氵 reminds you it's a sea.
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