Yep, this is for real. Portuguese Man o' War. A lifeguard put up this sign no more than 10 minutes after we arrived. Kailua beach is beautiful, and we hadn't been in the water yet, so we risked the jellies and went for a dip.
After we'd been in for a while I felt a stinging, burning sensation on my left hand. I instantly thought of the Man o' War warning and started looking in the water around me. Sure enough, a small Man o' War with a head about the size of a ping-pong ball and tentacles no longer than six inches was floating in the water right where I expected it to be. Fortunately, the sting was extremely mild. I didn't need any treatment, but I count myself blessed.
We've since been back to Kailua beach park, and we saw a few more Man o' Wars but none in our party were stung. We were approached, however, by a lady whose hand got stung and were able to point her in the direction of the lifeguard tower.
I guess even paradise has its downsides. Still, I'd prefer the risk of an occasional Man o' War sting to the guarantee of another stinging, biting, months-long winter on the mainland.