Simply meaning 'bad water,' the phrase 'agua mala' is used locally to describe most any kind of stinging thing that floats along this part of the Caribbean, including jellyfish.
For the first time recently I saw several of another kind of agua mala: Portuguese man o'war.
There were no live ones visible while I was in the water, I'm glad to say; but several had washed up along the beach to die. Their stings, leaving red welts that can last for days, rarely cause human death; but the venom causes intense pain.
This particular agua mala, the Portuguese man o'war, is a common part of the diet for loggerhead turtles. It's said that when you see lots of these around - in or out of the water - the turtle population is down; and we know that turtle populations are down world-wide.
The fact that they eat these things is one more reason among many I can think of to not kill sea turtles.