Always, always place cities on the coast if you can. The greatest reason is for trade routes. Cities on the coast automatically connect with eachother without roads. In addition, there's certain very valuble improvements only available to coastal cities; e.g Harbour, which increases trade route income by +50%. Having cities by the sea also gives you more choices for coastal wonders like the Great Lighthouse (+2 traderoutes in all coastal cities). What else... four resources can only be found off the coast (clams, crabs, fish, whales) and if you've got a good bit of culture over water, you increase your chance of getting an offshore oil well inside your borders. Finally, you have more production points for your navy.
<3 coast.