The other half of the water
There are two kinds of AIS transmitter at sea. Class A is the big, expensive, legally required one: cargo ships, tankers, ferries, anything over 300 gross tonnes on an international voyage. Class B is the small, cheap, mostly voluntary one: pilot boats, tugs, workboats, launches, a lot of fishing boats, and essentially every yacht that carries AIS at all.
They don't share a message. Class A broadcasts message 1, 2 or 3, the one our feed has always called "PositionReport". Class B broadcasts messages 18 and 19, which are different messages with a different name, and we never asked for them. Not on the Elbe, not in Rotterdam, not anywhere. So for as long as this world has had ships on the water, roughly half the traffic in every harbour has been invisible. Not through bad coverage or bad luck: we simply never put it on the list of things we wanted to hear.
That list now includes both class-B position messages and message 24, the static report that says what a small boat actually is.
The quiet ones arrive quiet
Message 18, the one most small boats send most of the time, carries a position, a speed, a heading and nothing else. No name, no type, no size. The boat is there and says nothing about itself for up to six minutes, until its static message comes around.
That gap is a privacy question rather than a rendering one. A hull we can't classify is far more likely to be somebody's boat than a company's ship, and this world has a standing rule about somebody's boat: sailing and pleasure craft are drawn, but never named, never written down, never collectible, because a name plus an MMSI plus a track is a person's movements rather than a company's logistics.
So an unclassified class-B contact starts on the private side of that line. It appears on the water as a small craft with a flag and a course and no identity, it can't be photographed for a card, and nothing about it is persisted. When its static message lands and says "tug" or "pilot boat", it becomes an ordinary vessel with an ordinary name. When the static message says "pleasure craft", it stays anonymous, as it always would have.
The error is cheap in one direction and permanent in the other. Anonymising a workboat for six minutes costs nothing; publishing an owner's name once can't be taken back.
What you will notice
More boats, mostly small, mostly close to shore, mostly in places that already had traffic. Harbours will look busier because harbours are busier than we have been drawing them. The launches and pilot boats that were missing are exactly the ones that move around a working port all day.
A few of the new arrivals will be photographable, since a class-B tug or fishing boat is as collectible as any other. Most won't be, because most of them are private, and that hasn't changed.
None of this fixes coverage. The feed still hears only what a volunteer receiver somewhere heard first, and there are real holes in that network. The Lower Mississippi and the Bay of Fundy are two we measured this week and both are still empty. Class B adds the boats that were being filtered out on our side. It can't add the ones nobody is listening for.