For the dressing: Smash the garlic cloves, sprinkle with salt and with the flat side of a large knoife, mash and smear the mixture to a course paste. Put in a bowl and whisk with the mayonnaise, vinegar and black pepper.
For the salad: put the water, garlic and salt in a large saucepan and bring to a boil. Add the carrots, lower the heat, and cook until the vegetables are tender but not msushy, about 5 minutes.
Stir in the radishes, then immediately drain all vegetables in the colander in the sink. Remove and discard the garlic.
Cool slightly and toss the vegetables with the dressing. Cover and refrigerate for 30 minutes.
About 10 minutes prior to serving, toss the cucumber, tomatoes and scallions in a small bowl with salt and peper to taste. When ready to serve, fold the mixture and herbs into the potato salad and serve.