Seiches

This phenomena is basically a standing wave in an enclosed/partially enclosed body of water. It can seem to be lunar tidal related but I don't think it is. This phenomena is equivalent to the sloshing of water in a bathtub and there is a nice diagram explaining the phenomena here. It is basically the sloshing around of water in an enclosed area due to prevalent winds pushing water in one direction. Once the wind stops blowing the water returns from the direction it came from. The movement can seem tide like but it isn't directly related to the sun or moon, such is the case with neap and spring tides. As always, there is more information available from wikipedia.