I've been playing for a while, and one house rule that frequently needs definition is what are and aren't "coastal areas"
(As an aside, the ACEP rule book defining coastal areas as only those that you can build or maintain ships with, which makes this whole discussion different... I don't know if that is intentional. But that's another topic

)
FYI, I'm using an electronic copy of the AdvCiv rulebook to quote from.
] 4.24 Coastal areas are areas which contain both land and ocean
] (nonlake) water.
Of course, the term "ocean" isn't defined anywhere

.
In the groups I've been in, we decided that a non-lake is any body of water with which a ship can give you movement possibilities that you do not have without a ship. That gave the groups I've played with that small tiny water area in the Assyria/Babylon area as a "ocean". Which meant that cities (generally wilderness ones near there) would be classified as coastal cities and vulnerable to Flood (rarely) and Piracy (more often).
I played with CraigB last month and he was using the definition that coastal areas are those sharing a boundary with an open-sea area (except for the ones in the NE corner which are defined as coastal.) I need to look at a map to see what precisely are all of the differences between the two interpetations.