It took me a while to figure this out.
You'll have to build one.
Plop down a lot, then make it commercial and categorize is as a Port.
Make a dock. To get the dock at the right level, start in the ocean and move the dock foundation towards the land. You'll also have to have a foundation leading to the dock with stairs, otherwise the Sim can't get off the dock, even though it might look level with the land.
Then at the end of the dock you need a Gangplank, an item available in the Commercial section of build.
That's the basics. You can fancy it up a bit with lights, benches, trees, etc. But the dock, foundation with stairs, and gangplank are all you need to make it work.