All of the above posts are accurate regarding the function of an anode on an outboard. There is however an additional possibility that can be in play.
If you have not noticed any previous degradation of the anode, and only saw a change when the boat was left in the water at "the camp", you may be seeing the result of a stray electrical current.
If your boat was left overnight at a dock or boat house that has any shore power, an improperly grounded AC line can also cause the "weak link anode" to sacrifice and exhibit the condition your photo shows. Stray current in the proximity of a docking facility with improper grounding, has eaten many an aluminum boat when the anode is completely consumed, not noticed for replacement, and then the aluminum hull itself gets eroded. Tiny pin holes are the first to be noticed. Also can be as simple as an improperly grounded battery charger utilized while the boat is in the water.
Might be minor on the short term, but if you plan on long term leaving the boat in the water at the camp, and the anode starts being aggressively consumed, have the AC ground checked. Stray AC current in proximity to water is why GFI's are required in wet locations.
Or it may be that you are just lucky enough to have salt water under your hull.
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