Shake table testing is a viable validation tool for bench-marking the analytically computed seismic response of under-water structural models with fluid–structure interaction effects. Conventional displacement sensors like LVDTs (linearly variable differential transformer) cannot be used for under-water response measurements as they require a stationary platform near the measurement point as a reference. This rather limiting application of LVDTs in several situations warrants the use of high speed camera as a potential measurement tool. This article presents the vision based displacement measurement of an under-water model tested on a shake table. High speed video camera is used to record the motion during shaking. The video images are processed using a special motion tracking algorithm and displacements are measured. The sloshing effect of water on the test model and their free vibration at the end of the shaking are studied to calculate natural frequency and damping. Vital information on the allowable under-water motion of the test model is obtained and discussed.