If your ship fills with water and sinks, does it flounder or founder?
How to remember it:
When something founders, it loses its foundation. (Founder and foundation have the same root.)
To founder is to collapse, sink, or fail.
One source of confusion here is that the meaning of the verb flounder is similar: to flounder is to struggle to move or get one's footing, or to proceed or act clumsily or ineffectually. People can flounder, but ships founder.