counter for small boats
艘 is a specialized counter used for small boats, treating each vessel as a discrete unit to be counted.
艘 combines 舟 (boat) with 叟, which likely contributes the sound. The character was created to serve as a counter for boats.
The left side 舟 is a boat, and the right side 叟 looks like an old man with a measuring stick. Imagine an old harbor master using a stick to count each small boat: one 艘, two 艘...
For ショウ, picture the old man counting boats and giving a short, sharp 'show!' command each time he points: show -> ショウ, counting boats one by one.