Diagnosis
Dirt, sand, or dried liquid (like soda) has gotten under the silicone membrane covering the button, gumming up the mechanical switch beneath.
How to Fix It
Clean the button area thoroughly. Unplug the speaker. Use a cotton swab dipped in 90% isopropyl alcohol. Wring it out so it's damp, not wet. Rub firmly around the edges of the power button silicone cap. Press the button repeatedly during cleaning to work the alcohol in. Wipe dry with a clean swab.
If That Didn't Work
The tactile micro-switch under the silicone is failing. This is a sealed unit. You cannot replace just the button. The entire top panel assembly would need replacement. Contact Bose for repair assessment.
Heads Up
Never submerge the speaker to clean it, even though it's IP67. High water pressure can force contaminants deeper into the button seals. Always clean ports and buttons with the unit off and dry.