It's a roughly accurate measure though. If there's a storm that slows them down, it would be perfectly acceptable for them to burn more fuel to arrive in port on time.
I think you are wrong about this, there are so many variables and sea conditions can affect things, for example older engines and maintenance at sea. AIS data is also known to be wildly inaccurate.
Source: built an engine performance model at a maritime company.
Too easy to fake. Maybe instead base it off of fuel consumption. At the end of the day, they can't fake a full tank of gas -- Require it to be documented when they buy fuel and use that to approximate what they're using for a cross-seas voyage (giving appropriate margin so that in case of storm/emergency they can still make it, but perhaps tax them if they eat into buffer)
Actually laughing at this, you have no idea, the people onboard type in the engine data and usually they type the exact same thing as the last reading with some decimal points changed and 0 if they were stopped (even though ships use plenty of fuel even if stopped usually).
It's the ships speed relative to the water what causes any fuel consumption. For the purposes of saving fuel, if you don't know the water surface speed, you know nothing.
Precisely! You could be going slowly against the tide or current or weather and using loads more fuel - you ask anyone who has been at sea about weather and localised conditions that don't show up on satellites are common enough for me to believe the on ship reports.
You should be able to accurately estimate all ships's speeds, using basic high school physics, from a site like https://www.marinetraffic.com/en/ais/home/centerx:-12.0/cent...