It is important to inspect the anodes after shifting waters and if necessary also clean the anodes. The anodes can also be pacified just by being away from water. If the drive has been tilted or for example placed on a trailer for some time make sure to take a look on the anodes.
If an anode for example looks yellow or is covered in white crust it has been pacified and needs to be brushed or changed to provide protection. This can be done by brushing the anodes using sandpaper or an emery cloth. Note, never use a wire brush with steel bristles. You have to use an emery cloth without iron oxide otherwise the anode might be pacified.
Make sure to inspect the anodes on a regular basis and change them if more then 1/3 has been eaten away by corrosion. All anodes do not share the same quality! Always use anodes produced by Volvo Penta since they have been tested to ensure maximum protection on stern drives and props.