Choose plastic watering cans, rather than metal, as they are lighter and have smoother edges so are less likely to bruise your legs.
Narrow-profile rectangular watering cans can be held closer to your body than round ones, which means you are likely to have a better posture and carrying a full can will cause less strain. Try carrying two smaller watering cans, instead of one big one, to distribute the weight more evenly.
Cans with a control valve in the spout, which is operated using a button in the handle, regulate the flow of water. These are very useful for all gardeners, but are particularly good for people with sight loss, if you use one hand or if you have difficulty bending.
Fit an outside tap, and/or a raised waterbutt which is connected to the drainpipe, to save on trips into the house.