sampan
舢 refers specifically to a sampan, a small flat-bottomed wooden boat. The meaning is narrow and concrete, centered on this particular type of vessel.
舢 is a phono-semantic compound: 舟 (boat) suggests the meaning, while 山 (mountain) likely hints at the sound. The character was created specifically for 'sampan', a loanword from Chinese.
A boat (舟) shaped like a mountain (山) — flat and wide — is a sampan.
For サン, imagine a sampan floating on a calm sea under the sun: sun -> サン, and the boat is a sampan.
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