Woody mediterraneun herbs like thyme, sage and rosemary don't need lots of watering. Soft green herbs like chives, parsley, basil, dill, French tarragon and coriander will need more water. If you grow mint put it in a pot and plant the pot as it's a bit of a thug. I think if you want comfrey I would plant it elsewhere, not in a herb bed. It's very vigourous and the roots are very deep. You could plant edible flowers too like nasturtiums, calendulas, pansies, borage and cornflowers. The blue flowers of borage are pretty in ice cubes.