Sinnakarachchi is the name of the lagoon of Nilaveli. It's situated to the north of Trincomalee. The famous holiday destination Nilaveli Beach is a 7.5 km long and 1.5 km wide spit of land in between the ocean and this Sinnakarachchi Lagoon, which opens to the sea north of Nilaveli, just below the bridge connecting the villages of Irakkakandi and Kumpurupiddi. Actually, the former village is also namegiving to the lagoon of Nilaveli, as Sinnakarachchi Lagoon is sometimes called Irakkakandi Lagoon. The mouth of the lagoon near Irakkakandi village is not far away from the maritime National Park of Pigeon Island. Boat tours to all maritime places of interest near Nilaveli - Pigeon Island, Coral Garden plus Red Rock, and Sinnakarachchi Lagoon - are offered by the Nilaveli Tourist Boat Services Cooperative Society Limited, which is a registered body of boat owners in and around Nilaveli. They also organize dolphin and whale watching tours and deep sea fishing. Their office is at the beach site of Nilaveli.

Sinnakarachchi Lagoon has two separate major arms, covering 780 hectares altogether. The main part behind Nilaveli Beaches stretches 9.5 km to the south in a slightly curved way. An even more curved northern arm stretches 5 km inland. The calm lagoon, which is connected with the ocean only by a small natural canal, is used as a sheltered anchorage for fishing boats.

With a maximum depth of 2 m, Sinnakaracchi Lagoon is fairly shallow and its water is more than brackish, actually it is categorized hypersaline. At several places along the lagoon's shores, crystallization of salt can occur during the dry season naturally. However, due to ist high salinity, salt pans for salt production were developed additionally, similar to those in Puttalam Lagoon at the west coast. Nilaveli Beach holiday makers interested in seeing salines, however, should travel 2 km further north to Periyakarachchi Lagoon near Vallipunam, where more salt pans can be seen, very close to the main road to Pulmoddai (B 424).

Due to ist mangrove swamps, seagrass beds and inter-tidal mudflats, Sinnakarachchi Lagoon is an important natural habitat of Sri Lanka's Eastern Province, particularly for waterbirds. However, the eco-system is threatend by the development of new aquaculture, whereas small subsistence fishery is the traditional way of using the lagoon. Wild elephants are known to frequent the area to the west of Sinnakarachchi Lagoon.

Boat services to the wetlands of Sinnakarachchi Lagoon start near Pulmoddai road (B 424) of Nilaveli Beach.

When approaching Colombo International Airport from the open ocean, aeroplanes cross the Negombo Lagoon just before landing. The lagoon, situated south of Negombo town and west of the airport, is 10 km long from north to south and 3.5 km wide on average. The palm-rich spit separating it from the ocean is only 500 m wide on average, at some places only 250 m. The southern end of the estuarine lagoon is the sanctuary of the Muthurajawela wetland in the river delta of the Attanagalla Oya. This mangrove swamp attracts a wide variety of water birds including gulls and other shorebirds.

The Negombo fishing harbour is situated at the northern end of the lagoon, where it opens to the ocean via two narrow channels.  The lagoon is used for fishing, too. Traditional Oru boats, sometimes wrongly called katamarans, can be seen on the ocean and in the lagoon alike. The crabs and prawnd of the Negombo lagoon are held in high esteem for their taste. Motorboat tours can be arranged at the fishing port as well as in the Muthurajawela wetlands. Oruwa “katamaran tours” can be organized, too.