There are at least two of these sea-ports in my San Francisco FS Sim neck of the woods...

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Commodore Centre: 22CA, upstream of Golden Gate bridge in Marin County.
Also, Lost Isle: Q87, located on the Sacramento River, upstream of Concord Buchanan.
Beware tho'....
If you select one of these ports from the drop-down menu with your wheeled Cessna 172 you will promptly be dumped into deep water, and therefore sink without trace...

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Make sure you have some floats attached to the underside of your trusty aircraft...

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LOL...!
Paul....floating somewhere...

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Note: For users of FS Navigator the water "airfield" Icons appear different to the land airfield Icons...

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.