Capt. Ken Henry, AMS, IIMS, IAAI
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Replace the zincs every year.  If you have excessive pitting or bleaching of the hull, you have a stray current issue, which may be caused by your boat, neighboring boats, or marina.  
Each year, check the boots and hoses for your outdrives.  The boots and hoses should be replaced every five years.  If you notice a pin hole or any deterioration of the rubber, replace immediately.
Check your shore power cord(s) and inlet(s) for signs of overheating.
Inspect the hoses on the engine(s). If the hoses are showing cracks, it is time to replace. Use the right hoses when replacing. Premium water hose (green label) can not be used for exhaust hose (blue label.) Also, if replacing exhaust
hoses, each end will need to be double hose clamped. The hose clamps need to be staggered.