Why does Santiago hate the Portuguese Man of War?

The Portuguese man-o-war was posionous.

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.