It's a good question that's sure to get answered after it swoops out of the strategy articles forum by a moderator.
My answer is that a coastal city, as it pertains to the Great Lighthouse, is one which has access to the ocean. If you have an interior port on the water with access to a lake (freshwater, two food + commmerce tiles), it can build naval units, but it is not GL worthy. But, I don't know the bounds are when a freshwater lake becomes ocean, because sometimes you can have ocean tiles if the "lake" is big enough, even if it is surrounded by land. In the event of this kind of ocean access, I believe it will benefit from the Great Lighthouse.
