In a large saucepan, combine 6 cups of the water with the white wine vinegar, sugar, peppercorns, garlic, sea salt and fennel seeds and bring to a boil over high heat. Boil vigorously for 2 minutes, then add the fennel wedges and cook until just tender, about 5 minutes. Using a slotted spoon, transfer the fennel to a large bowl. Add the zucchini to the vinegar mixture and cook until crisp-tender, about 1 minute, then add to the fennel in the bowl. Repeat the process with the cauliflower, cooking it for about 3 minutes, and then the red onions, cooking them for about 2 minutes.