Cerberus schneiderii
Singapore (© budak) 
Singapore (© Gabri Mtnez) 
Singapore (© Daniel Rosenberg) 
Singapore (© Low Choon How)
English name: Dog-faced Water Snake (aka “Southeast Asian Bockadam” or “Schneider’s Bockadam”)
Scientific name: Cerberus schneiderii (formerly part of Cerberus rynchops)
Thai name: งูปากกว้างน้ำเค็ม (Ngu Pak Kwang Nam Khem)
Description: To 120cm long. Relatively slender, cylindrical body for a water snake. Head is wider than neck with thick fleshy cheeks. A dark eyestripe passes through the relatively small eyes. Body is brownish or grayish with dark crossbars, with a venter that is mottled gray and cream.
Relevant scale counts: Usually 23 midbody scale rows of keeled scales (rarely 25). Only the last upper labial is horizontally divided and the nearby scales are smooth.
Similar Species: South Asian Bockadam usually has 25 midbody scale rows, the last two upper labials are horizontally divided, and there are keeled scales in front of the jaw angle
Jack’s Water Snake is dark brown with narrow light markings rather than lighter brown-to-gray with narrow dark markings and lacks the horizontally protruding eyes.
Puff-faced Water Snake is dark brown with narrow light markings rather than lighter brown-to-gray with narrow dark markings and lacks the horizontally protruding eyes.
Keel-bellied Water Snake has a lighter cream-to-yellow body with no clear dorsal/ventral contrast.
Bocourt’s Water Snake has a thicker, darker body and typically is only found in freshwater settings.
Habitat: This species is found in mangrove mudflats and brackish river mouths, only occasionally venturing upriver.
Place in the ecosystem: Eats fish. May be eaten by larger snakes, large fish, monitors, and wading birds.
Danger to humans: The Dog-faced Water Snake is rear-fanged but its venom is not a threat to humans.
Conservation status and threats: In the past hundreds of thousands of these snakes were collected for their skins, but the species still appears to be quite common where found.
Interesting facts: Dog-faced Water Snakes have been shown to have a salt gland that excretes excess salt out of their bodies. This would enable them to survive in saltwater environments for extended periods of time. However, they still take the opportunity to drink freshwater when it is available.
The Dog-faced Water Snake can be incredibly prevalent in the mangrove mudflats. During daytime none of the snakes will be visible as they hide in crab holes or under water, but in the night at low tide they will completely cover the landscape in the right habitat. One study in Malaysia found a Dog-faced Water Snake occupying every 1-3 meters of shoreline.
References:
The Dog-faced Water Snakes, a revision of the genus Cerberus
IUCN Redlist: Cerberus rynchops
A Checklist and key to the Homolopsid snakes
Wild Singapore: Dog-faced Water Snake
Ecology Asia: Dog-faced Water Snake
Diet, feeding behavior, growth, and numbers of a population of Cerberus rynchops
Evolution in the Mud
Photographic Guide to Snakes and Other Reptiles of Peninsular Malaysia, Singapore and Thailand
Reptiles and Amphibians of Thailand 
saw one in my garden in Sa Kaeo . No real water nearby but always some smaller sources .Didn’t know what it was at first but knew it wasn’t a viper a sure it was not a cobra . Now rarely see snakes , a shame .