An offshore intake structure for a water cooling system having an intake shaft communicating with a lake or like reservoir comprises a well excavated in the bed of the lake having an opening communicating with the intake shaft. The well is covered by a submarine roof structure having a perforate outer region defining an inlet to the well and an imperforate inner region which extends over the intake shaft opening and radially outwards therefrom so as to define with the well bottom a horizontal flow passage extending inwards to the intake shaft. The construction is such as to permit an adequate intake flow while keeping the intake velocity below the limit at which fish would be entrained.