Trim the shallots and break the cauliflower into small florets. Core and deseed the pepper and cut the flesh into 2 cm (¾ inch) squares.
Put the water and vinegar into a heavy-based saucepan, bring to the boil and add the cauliflower, pepper and shallots. Return the liquid to the boil and boil for 2 minutes. Take the saucepan off the heat and leave the vegetables to cool in the liquid.
Trim the beans and sugar snap peas and blanch in lightly salted boiling water. Refresh them in cold water and drain.
When the pickling liquid is cool, strain the vegetables and mix them with the beans, peas and watercress in a large salad bowl. Dress with olive oil, season to taste with salt and pepper and serve.