Abstract: A sub-structure for boat docks consisting of an elongated, ladder-like frame having a pair of side bars connected by a series of cross bars extending between and connected at their ends to the side bars, the side bars and cross bars being of such cross-sectional configuration, and the connections therebetween being of such a nature, that the frame may yield in a vertical direction with relatively great resilient flexibility, whereby to accomodate wave action in the water therebeneath, while at the same time being relatively highly resistant to deformation in its own plane for maintaining its horizontal form and shape. Generally, the side and cross bars are of such configuration as to strongly resist bending stresses, but to permit torsional yielding with relative ease, and the cross bars are connected to the side bars to permit a degree of universal pivotal movement therebetween.