A hydrostatic load is useful to counterbalance the weight of the hull, which means a Gravity load must also be present. If you have the correct draft line, the net upward component of the hydrostatic load will exactly match the weight of all the parts in the model plus the crane weight load represented as a downward force.
Even when you have a perfectly balanced set of loads, Static Structural needs a stiffness matrix that is not singular. That is done by constraining all six DOF of rigid body motion. You are doing that with the two remote displacements. In this case, the reaction force will be close to zero.
I don't know how global bending moments for ship hulls are computed and what conditions they apply to. Are they intended to replace using the loads I described in my first paragraph?