Experiments you can run yourself.
Archimedes' principle and the floating condition: why a steel ship floats. An interactive simulation of the buoyant force, body density vs fluid density, and the formula F = ρgV.
Hydrostatic pressure depends only on depth and the density of the liquid, not on the shape of the vessel. An interactive simulation, the formula p = ρgh and connected vessels.
A small force lifts a large load. Pressure is transmitted equally.
The same liquid settles at one level regardless of the shape of the vessel.
In a pipe constriction the fluid flows faster and the pressure drops.