When the floating part of a wave energy converter (WEC) is constrained only by a flexible connection to the power take-off unit or sea bed mooring point, the float will move in several modes including vertical (heave) and horizontal (surge). It is important that the displacement magnitudes in these directions are quantified through experiments to evaluate the accuracy of numerical predictions of device response and mooring line loads. In the case of multiple devices, horizontal motions may be large enough to change the spacing between devices whichcould modify the response, and therefore array layouts must be designed to avoid contact between adjacent floats. In this paper experimental data is reported to inform the development of numerical models of response for wave energy converters and floating bodies which include a flexible constraint. Usinga combination of measurement and observational techniques, distinct steady-state motion modes which are influenced by the incident wave frequency and simulated level of power take-off are reported for a single device and two closely spaced devices. The occurrence of parametric excitations is also highlighted forparticular combinations of incident wave frequency and the level of simulated power take-off.