Which creature does Santiago hate and why?

The old man hated the Portuguese man-of-war. From the text....

The strange light the sun made in the water, now that the sun was higher, meant good weather and so did the shape of the clouds over the land. But the bird was almost out of sight now and nothing showed on the surface of the water but some patches of yellow, sun-bleached Sargasso weed and the purple, formalized, iridescent, gelatinous bladder of a Portuguese man-of-war floating dose beside the boat. It turned on its side and then righted itself. It floated cheerfully as a bubble with its long deadly purple filaments trailing a yard behind it in the water.

“Agua mala,” the man said. “You whore.”

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They were immune to its poison. But men were not and when same of the filaments would catch on a line and rest there slimy and purple while the old man was working a fish, he would have welts and sores on his arms and hands of the sort that poison ivy or poison oak can give. But these poisonings from the agua mala came quickly and struck like a whiplash.
The iridescent bubbles were beautiful. But they were the falsest thing in the sea and the old man loved to see the big sea turtles eating them.