Would a boat floating in a sea have more or less upthrust than the same in freshwater?

Answer 1

Assuming its weight hasn't changed, then as it is floating, the upthrust on it equals its weight and so is the same.

What WILL change, however, is how deep in the water it floats. The upthrust is provided by the weight of the displaced volume of water, the volume of which depends upon the density of the water which depends upon the temperature and saltiness (amongst other things) of the water.

As freshwater is less dense than [briny] sea water, it will require more volume for the same weight and so the ship will float deeper in freshwater than in [briny] sea water. This can be seen in the load lines on the "Plimsoll Line" where the Freshwater line is much higher than, say, the Winter North Atlantic line (the latter being more salty and colder, and so much more dense) meaning that for the same load, that the waterline in Freshwater will be much higher up the ship than in Winter North Atlantic (or alternatively, the ship will float much deeper in the Freshwater than the Winter North Atlantic).