Up to you. Technically, fresh water requires a magnesium anode and salt water requires a zinc anode.
Having electric start has nothing to do with corrosion. Corrosion is caused by two dissimilar metals in close proximity to each other or touching each other in a conductive medium. Salt water is a very good conductor and what happens is that by placing your engine in salt water the salt/aluminum and stainless steel is creating a battery with the aluminum castings being the sacrificial anode to the stainless steel parts.
Short answer is yes. Place at least a zinc on the gearcase. Keep the zinc clean, you will notice that a white coating will start to cover the zinc. Try and keep that at bay. What you want is a zinc that is sacrificing itself, not one that looks good!
As far as not needing one if you trailer? You decide. While it is in the water it needs it. Zincs are inexpensive and replaceable.
