Abstract: A sub-tidal platform adapted to be placed under water in front of a beach comprises a support structure having at least two upwardly extending, spaced-apart side walls extending along vertical planes. Each side wall has a bottom adapted to rest on a generally horizontal surface, and a sloping, upwardly extending edge which is at an acute angle with respect to the bottom. A plurality of interconnecting members extend between the side walls for maintaining them in an upright position. The interconnecting members are positioned adjacent to the sloping edge of the side walls. A plurality of gates are pivotally connected to the interconnecting members of the support structure for controlling fluid flow through the space between the side walls. The gates open in response to incoming fluid flow through the gates, and close in response to outgoing fluid flow in the opposite direction.