Be AWARE---there is another more serious issue that zincs/magnesium SACRIFICIAL anodes will protect against--stray 12Vdc current and floating AC grounds that are more and more common in public marina's and facilities because people have added lots of creature grounded comforts and electronics to thier boats.
If your boat isn't absolutely protected by sacrificial anodes, Galvonic Isolators, and an absolutely perfect system BONDING system your drives, Gimble's, Bellhousings and more can be destroyed EVEN in a weekend. Your insurance proabably excludes damage from electrolysis--like my State Farm policy.
This kind of damage can "git" everything in/on your boat !!!
Apologies for ranting but I got this F$$KING this winter in less than 4 weeks !!!