[masraum]

Originally Posted by island911

These ^

Some of the first gravity experiments were done against a mountain (to Frost's point) to measure the affect of mass.

And relative specific gravity of steel is 7.8 (to hbueno's point) meaning that the weight of the steel will be ~7/8ths of what it is on land.

Good call, relative density/specific gravity of steel at 7.8

RD = weight of the object in air / weight of object in air - weight of object in water

Of course, the specific gravity of salt water is different from freshwater. Also I believe temperature makes a difference, but that's probably a good start.

If the difference between sea level vs top of Mt Everest is only 0.25%, then I suspect the difference at the bottom of the Mariana Trench would probably be similar, so yes, the boat would weigh less at the bottom of the trench based on specific gravity of steel, and probably even less due to the salinity of the sea water.

https://www.britannica.com/science/seawater/Density-of-seawater-and-pressure
It looks like the density of sea water @ 10km is 1.07, so the steel would be about 87% of the weight in fresh water, but possibly another 7% lighter due to the salinity farther down, maybe more if it was even deeper.