screen door on a submarine

A screen door on a submarine would be absurd as it would allow the vessel to fill with water.

• "But realists put the final number of jobs saved through privatisation at no more than 800. "It's about as useful to us as a screen door on a submarine," says an angry mayor's office in Sacramento ."
• "I doubt anybody would give you static about unloaded and stored long guns anywhere in U.S. waters, and few places would about handguns (although unloaded guns are as useful as a screen door on a submarine).”"
• "If you don't know what security vulnerabilities look like at the code level, then having the source code is just as useful as a screen door on a submarine from a security perspective."