The Archimedes' principle states that the loss in the apparent weight of a fully or partially submerged body is equal to the weight of the liquid displaced. In an elevator accelerating up neither we get the true weight of the body nor the true weight of the displaced liquid. The weight of a body with mass m in the elevator going up with acceleration a = m(a+g) and the buoyancy force B = m'(g+a) where m' is the mass of the liquid displaced. So the Archimedes' principle is not valid.           

Let us see what happens in an accelerating car. The liquid and the body both will have a pseudo force in the opposite direction in this non-inertial frame. If the acceleration of the car is 'a' the weight of the body will be m√(g²+a²) recorded in a spring balance. The direction will be along the resultant of g and a. Similarly, the force of buoyancy will be m'√(g²+a²) in a direction opposite to the weight. So the Archimedes' principle is not valid even in this case.