How do the forces acting on a boat compare to the force acting on a plane ?

Upthrust is the upward force exerted on a body immerses in a fluid.

Gravitational force is the foce of attraction on a body on other bodies

The force acting on the boat is upthrust while the force acting on the plane is the boat is gravitational force

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.

it is a negative acceleration