small boat
艋 centers on a small boat, a compact watercraft used for transport or fishing.
艋 combines 舟 (boat) with 孟, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of small boat.
The left side 舟 is a boat, and the right side 孟 looks like a child 子 on a dish 皿. Imagine a small boat carrying a child in a dish-like hull: a tiny, compact vessel.
For ボウ, picture a small boat with a bow that curves upward: bow -> ボウ, and the little boat bobs on the water.