at a low ebb

The figurative sense is an extension from the literal sense of low tide.

at a low ebb

1. (idiomatic) At a low point; in a poor state; in a state of decline or depression.
   • 2022 August 24, Tassanee Vejpongsa, Grant Peck, “Thai court suspends PM Prayuth pending ruling on term limit”, in The Washington Post‎, retrieved 2022-08-24:

     Polls show the prime minister’s popularity is at a low ebb.

• ebb and flow
• low tide

• nadir

• “at a low ebb”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.
• “at a low ebb”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “at a low ebb”, in Collins English Dictionary.
• “be at a low ebb” in Longman Dictionary of Contemporary English, Longman.