The tanker weights exactly as much as the weight of the water that it displaces. They are in balance. You describe it yourself. The tanker sinks deeper if it becomes heavier and swims more up as it becomes lighter.
The measure of “boat swims” is not the weight of the displaced water. It is wether there is some boat wall left sticking out of the water to keep more water from entering and displacing the air that keeps the submerged volume in weight balance with the water.
Since we are pedantic, what you say isn’t true.
The tanker weights exactly as much as the weight of the water that it displaces. They are in balance. You describe it yourself. The tanker sinks deeper if it becomes heavier and swims more up as it becomes lighter.
The measure of “boat swims” is not the weight of the displaced water. It is wether there is some boat wall left sticking out of the water to keep more water from entering and displacing the air that keeps the submerged volume in weight balance with the water.
So we have rising sea levels because there’s so many big ships in the ocean, got it.
Xkcd did this one.
https://what-if.xkcd.com/33/