Starting with the basics:

Velocity is defined as displacement per time. There are various expressions of this, like dx/dt from calculus for example. Problems involving rates of change over time are dynamics problems. If the motion is one-dimensional or 'single degreee-of-freedom' then you do not need root-of-sum-of-squares position (RSS of x, y & z positions) to determine displacement, it is simply X2 - X1, or the difference between final position (2) with respect to initial position (1). So you need to define a coordinate system for your dynamics problem.

1st rule of dynamics, always define your coordinate system.

2nd rule, always develop a Free Body Diagram (FBD) of the object (or entity) in question as it resides within your coordinate system. This allows subsequent definition of the govern equation(s) of motion.

The FBD must identify ALL externally applied forces acting upon the object/entity. Then you apply Newton's second law to the FBD, F=mA, where F is the summation of applied forces, m is mass and A is resultant acceleration. F and A are vector quantities, m is scalar mass (magnitude only, no direction).

Dynamics problems are actually as simple as F=mA, but the correct setup is critical for success.

Choice of coordinate systems can simplify or overly-complicate the resulting equations. If you define the coordinate origin to be the initial position, it may simplify your governing equation. All of these considerations should be applied before any application of complex modeling tools such as ANSYS, in order to estimate an expected result and recognize unexpected results that need further explanation. Conceptual modeling must come before numeric, to avoid the risk of the common "garbage-in-garbage-out" scenario.

The plate speed of the computer model output shouldn't exceed the particle speed of the wave (typically less than wave speed), otherwise your model probably has violation of the laws of conservation in one form or another. This gets into the concept of wave motion, which is beyond what you are trying to do. In a fluid with harmonic wave motion like you are trying to model, particles typically oscillate in an orbital pattern as the wave passes through the propagating media (the water in your case).

If your plate is 'unrestrained' or 'unconstrained', it will give you one answer, but if you try to extract work from it by making it push on a piston or or some other form of mechanical energy conversion (actuator) for power extraction/generation, that restraint or constraint or impedance will reduce the resultant deflection of the plate. The force acting on the plate over the deflected distance give the product of work (W = F times D, when F and D are acting along the same parallel or linear degree of freedom). This product gives a theoretical maximum of the possible energy that may be extracted.

Your actuator (piston or generator, whatever it is) will have a conversion efficiency less than unity (