In Australia, one would not say 'I see the fisherman IN the harbour' as that would imply they were in the water. If they were in boats, then one would then say: 'I see the fishing boats in the harbour', not 'the fishermen.' If one is talking about seeing fisherman standing on the wharves fishing, that would be 'at' the harbour, not 'in' the harbour.
My understanding is that dei is used not only for 'of the' but used it is also used for for 'some'. Whereas 'i' would only be used for 'the'. So 'i pescatori' would be 'the fishermen' and 'dei pescatori' would be either 'some fishermen' or 'of the fishermen'. Hope that helps.