The present invention is a waterslide toy block construction system. The present invention includes a plurality of toy construction blocks that can be arranged to create a variety of multi-block structures. Some of the construction blocks have a plurality of elongated projections extending from said top surface, and the undersurface of such blocks defines recesses therein for frictionally engaging the elongated projections of other such blocks. Other construction blocks have conduits for water flow (hereinafter groves) which can be assembled to create continuous, extendable and generally horizontal and vertical paths for the travel of water thereon. The top surface of such blocks defines at least one groove thereon for the routing of water. When a user correctly arranges the blocks to form a multi-block structure, these grooves form a path thereon for the travel of water throughout such a multi-block structure. In this embodiment, there is also a construction block unit having mechanism for connecting a water outlet, such as a hose or a tank. The connection mechanism incorporates and adapter to reduce water pressure and to prevent detachment of the water outlet due to hydrodynamic motion therethrough.