I’m referring to this:
image: Harbor Island Marina in Port of Seattle (probable source: portseatlle.org)
I added a red arrow.
Boats lined both sides of the [. . .], bobbing quietly under the blue sky.
It’s a pier or a dock or a wharf.
In American English, a dock is technically synonymous with pier or wharf—any human-made structure in the water intended for people to be on.
The structure shown is clearly a human-made structure in the water intended for people to be on. Therefore, it is a dock in American English. Whether it is also a pier or wharf may vary depending on what part of the world you live in. Here it would also be called a pier.
Yes, it’s floating. But it’s still a floating pier or floating dock.
One word is jetty.
A landing stage or small pier at which boats can dock or be moored:
Ben jumped ashore and tied the rowboat up to the small wooden jetty
It appears that this is a British-English usage of the word and American English uses different words for various marine structures — or uses the same words in different ways. Despite that, Google images for jetty include the sort of thing illustrated in the question:
...and some rather larger structures:
Luke Roberts via Wikimedia Commons
I'd usually refer to them as a "pontoon".
Jetty, dock, quay and pier all tend to be non floating. The OP's original picture is a device that floats with the tide but is kept in place by the upright poles so the boats are always at the same level as the "artifical ground level" created by pontoon.
The floats under the decking are often referred to as pontoons (much like the pontoon on a sea plane) but the decking and floats combined are what I would call a Pontoon
It is variously a pier, dock, wharf, jetty, or pontoon, depending on the local dialect and sophistication of the speaker. Call it a "dock", and everyone will understand what you're talking about while only a few of them will yell at you for mis-using terminology.
English has an enormous and highly-detailed vocabulary regarding ships and their support facilities that developed back when travel by water was the best way to get around. The distinctions have been fading in the past century or so, and many formerly-distinct terms are used interchangeably.
"A pier is a raised structure, including bridge and building supports and walkways, typically supported by widely spread piles or pillars. The lighter structure of a pier allows tides and currents to flow almost unhindered, whereas the more solid foundations of a quay or the closely spaced piles of a wharf can act as a breakwater, and are consequently more liable to silting. Piers can range in size and complexity from a simple lightweight wooden structure to major structures extended over 1600 metres. In American English, pier may be synonymous with dock." "A jetty is a structure that projects from the land out into water. Often jetty refers to a pier, wharf, dock, breakwater." from wikipedia. so its a jetty and a pier/dock. hope that helps