You should wear the Actigraph on your non-dominant wrist (i.e. if you are right handed please wear the Actigraph on your left wrist).
The Actigraph is measuring how active you are by recording how much you move and sleep.
Yes, it records your movement during the day and your sleep at night.
Yes. The Actigraph is water resistant and can be worn in the shower, but it should not be submerged (please remove before swimming or taking a bath).