The caption on the sign didn't tell me much, so I had to climb up on, and balance atop, a bollard to get close enough to scan the QR code (which you can see at the bottom left of the sign in the previous photo) with my phone. Decoded, it read:
8 million swimming,
The traffic rolling like waves,
Watch for undertow.
It turns out that this sign is one of 200+ installed around the city (there are a dozen different haikus), intended to engage pedestrians and encourage safety. I was certainly engaged, although it's unlikely that perching atop a bollard to figure things out made me any safer.