[tjradcliffe]

For comparison, the Great Lakes are about 60 m above sea level (Kingston, at the junction of Lake Ontario and the St Lawrence, is at 200 ft). Salt water inundation has not been a problem. However, invasive species (zebra mussels, for example) have been. They are fresh water species that get sucked up into ballast water in other ports (many of which are on rivers or estuaries) and pumped out in other destinations. There are procedures to avoid this (exchanging fresh-water ballast for salt water while at sea, for example) but those procedures are inevitably implemented by humans, who are certain to eventually make a mistake sufficient to cause contamination.