The 800mm diameter High Density Poly Ethylene (HDPE) pipe laying was designed, fabricated and laid under water across Batticaloa lagoon South basin, to convey drinking water from Unnichchai to Batticaloa town. A trench formed under water using a cutter suction dredger in the canal bed to lay the pipe without it being affected by surface movements. The length of the trench was approx.01 Km and rock out crop was encountered in a stretch of approx.200m along the trench which had to be blasted underwater to reduce a rock formation to the designed level. The invert of the trench was filled with sand bedding to prevent damage to the pipe. Ballast were designed and fixed on to the empty pipe line to hold it down at the bed. Similarly 400mm diameter HDPE triple line was also laid across of the Batticaloa lagoon East basin.