even in a "hot" marina it takes at least a month to do any appreciable damage to our prop zinc. I've been in some where I had to replace the zinc every three months - Mexico used to be really bad about this, but has improved a good bit over the years. Alaska was another bad place what with all the commercial fishing boats constantly running big AC generators, and other equipment, without proper grounding devices - in many marinas it was hard to get very far away from them.