A Tsunami (Japanese for 'harbour wave') is a wave generated by an undersea earthquake,landslip or volcanic eruption so if you want to demonstrate what causes a tsunami rarher than it effect that it may have then we have to think about simulating an undersea earthquake.
I think that the easiest way is to construct a wave tank,as you considered.A good size would be about 70cm long x 40cm wide x 10cm deep.
Next make a flap from perspex or wood about 5cm long x 40cm wide (a little smaller than the width of the tank) x a few mm thick ,with a hinge fastened along the 40cm side.Fasten the hinge to the base of the tank at one end with the hinge furthest away from the end wall.Now connect a piece of thick wire to the flap so that the wire projects out of the water.Pulling the wire sharply should rotate the flap about its hinge.
Now with about a 5cm depth of water,when you pull the wire this should simulate a sea-floor disturbance which is what happens in a tsunami.
If you make the floor of the tank slope upwards so that,at the far end away from the hinge, the water level is only about 2cm you may be able to simulate the increase in wave height which occurs in real tsunami as the wave approaches the coastal shallows. Why not experiment with different size flaps and different water depths?