Only needed if you are leaving it in the water for any length of time, ie, on a mooring or in a pen.

 As ram are said.  No need unless permantly in water , the motor do have anodes mounted on them to prevent any issues when running each day and take care of stray currents from the boat and electrical system whilst out each day .most motors have a small sacrificial anode as a thrust skeg under the cavitation plate along with a anode that bolts across the transom bracket of the motor and sits in the water when at rest.

 i have been told the anodes on the motor are grounded to the vessel aswell and have some sort of effect resistng electrolises on the hull too?

not sure if this is true, is this becuase some boats may be wired differently using the hull as a ground and need the anode ?

 Anodes on the motor are meant for protection of the alloy parts of the motor itself, while they are submerged. You would be unlikely to get much cathodic flow between the aluminium of the hull  and the alloy of the motor cases, or SS prop. Just not that much difference in them. And remember, just as an aside, that while motors are grounded to the hull, that ground is not part of the electrical circuits of the boat, ie it is not actually carrying current. Aluminium huls should never use the hull as the negative, there should always be two wires to everthing. You can get away with it on cars,common practice, but cars ain't boats.