Pat the swordfish dry and season both sides with the pepper. Rinse the capers, halve the olives, and finely dice the onion and celery.
Heat the olive oil in a 30 cm / 12-inch deep skillet over medium heat. Add the onion and celery with a pinch of salt and cook for 6 minutes, stirring, until soft and translucent but not browned. Stir in the garlic and pepper flakes for 30 seconds.
Pour in the wine and simmer for 2 minutes until the sharp alcohol smell fades. Add the crushed tomatoes, water, olives and capers. Simmer over medium-low heat for 12 minutes, stirring now and then, until the sauce coats the back of a spoon.
Nestle the swordfish steaks in a single layer in the sauce and spoon some sauce over the tops. Cover and cook at a low simmer for 5 minutes. Flip, spoon over more sauce, cover, and cook 4 minutes more, until the fish is opaque and a thermometer reads 145 F / 63 C at the thickest point.
Taste the sauce and add salt only if it needs it. Scatter the parsley over the pan and serve right away, with lemon wedges.