One thing about sacrificial anodes is that multiple seem to be required. E.g. even though the hull of a ship may be a good conductor, you don't just have one sacrificial anode.
Wikipedia ["Galvanic anode"]: The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.2 mi) long needs 10 anodes, then approximately one anode per kilometere would be more effective than putting all 10 anodes at one end or in the centre.
I came here to say the exact same thing. The resistance of the metal surface will mean you want to cover the metal needing protection all around. I think dependant on the salinity of the water different metals should be tested to best suit you environment.
Wikipedia ["Galvanic anode"]: The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.2 mi) long needs 10 anodes, then approximately one anode per kilometere would be more effective than putting all 10 anodes at one end or in the centre.