We report experimental observations on the nature of eddies formed at sharp edges by the action of periodic flows and the influence that these eddies might have on the efficient operation of certain types of wave energy converters. Flow visualization techniques are employed to observe the interaction of regular gravity waves with immersed and semi-immersed flat plates. Similar techniques are applied to the example of a glass tube held vertically in a free surface. Here the special situation is examined when highly amplified flow-oscillations occur within the tube as a result of its particular resonant properties. It is found that the exit geometry is critical to the degree of eddy formation; when terminated with an abrupt exit, separation occurs both in the in-flow and the out-flow and a clearly defined succession of vortex rings is formed; when terminated with a flared bellmouth no such separation is observed and the flow remains well ordered through the cycle of oscillation. The energy losses associated with such flows are examined in detail, and from this the nature of the contribution from eddy formation is identified.