What doesn't need watering - most importantly the thing not to waste water on is a lawn, unless you are establishing a new lawn, here in the uk, even in the driest summers the roots of the lawn will stay alive, and when you get rain again it will green up.
Borders, - as long as your plants have been established more than a year, very few border plants will need watering unless we have a prolonged period of dry weather (more than 2 weeks). Save your bath water to spot water hydrangeas, roses, clematis and fruit trees.
Vegetables & fruit, - this is where to use all your waterbutt water on, water each plant or row of seedlings by using a gentle stream from a can nozzle to the soil at the base of the plant, - gentle so as not to wash away the soil from the roots and slowly so as to ensure that water goes deeply down to the root system of each plant rather than soaking away on the surface.
It is better to water deeply once every 3 days than with a sprinkling of water every day. Early evening is a great time to water, as less water will be lost in evaporation
Plants in pots, - these will rely on you for water and how often they need water depends on the size of the pot, the type of compost and whether they are standing on a tray- at this tiime of year get everything potted up into as big a pot as possible and use watergel for your hanging baskets and pots.
It's commonly believed that you shouldn't water during the day if it is sunny, however if a plant needs water, don't ignore it to the evening, - it is ok to water during the day, but not ideal -don't get water on the leaves, as that may cause scorching.