Fuel consumption in boats is usually measured at some speed relative to the waters surface, not the earth.

That doesn't make consumption double for "traveling upstream" vs "traveling downstream". The "traveling" part means actually getting somewhere, not treading water. If you turn off your engine entirely, you will still travel downstream and consume zero fuel. The article is about the necessary specific context for the statement to be accurate.

There's a point about fuel consumption when travelling downstream though: you can't just switch off your engine and drift downstream, because you'll run aground fairly soon. The ship needs steering, and steering needs engine power and relative speed to water.

Or relative to time (litres or kilograms per hour).

Time on it’s own is not enough as the faster the boat goes the more fuel it consumes per hour.

That said, at each specific speed you get a rate as x gallons per hour.

prob a journalistic typo.

I'm not sure if this is a typo, but you need a certain speed to be able to steer the ship. Going too slow downstream is like stalling a airplane, you lose control over it.

This relies on the surface of the water being static. In most rivers the surface flows. Indeed, there's a shear layer under the surface, so in a river these effects happen in the opposite direction.

has to be a typo