Homalopsis mereljcoxi
Nakhon Ratchasima (© Alex Heimerdinger) 
Thailand (© John Murphy) 
Bangkok (Jon Hakim / CC BY-NC-SA) 
Pathum Thani (© Michael Cota)
English name: Jack’s Water Snake (aka “Jack’s Masked Water Snake”, “Puff-faced Water Snake”)
Scientific name: Homalopsis mereljcoxi (formerly Homalopsis buccata)
Thai name: งูหัวกะโหลกไทย (Ngu Hua-kra-lok Thai)
Description: To 137cm long. Robust, somewhat flattened body. Notable broad, brown head with dark eyestripes, a “V” marking on top of the head and an inverted “V” on the snout. Body has brown bands (often outlined in black) alternating with narrow light bands that fade to brown in old age. Underside is white to yellow with small black dots.
Relevant scale counts: 40-49 midbody scale rows with keeled dorsal scales, single frontal, 2 postoculars/1 postsubocular, usually more than 165 ventral scales.
Similar Species: Bocourt’s Water Snake has no face mask, is thicker and darker with black markings interspersed with the brown.
Red-tailed Pipe Snake has a small, dark head, smooth scales, and a barred underbelly.
Dog-faced Water Snake has black bars that are narrower than the light background and has dorsolaterally protruding eyes
Deuve’s Water Snake is darker, has a dark venter with pale dots, and 35-40 midbody scale rows. It is only found in the Mekong River drainage.
Puff-faced Water Snake has slightly broader bands with less contrast, 33-40 dorsal scale rows and usually fewer than 166 ventral scales. It is only found in south Thailand and countries to the south.
Martaban Water Snake has black-outlined saddle-shaped blotches rather than bands, usually a divided/broken frontal, and fewer than 165 ventrals. It is only found in coastal Myanmar and southern Thailand.
Range: Thailand, Cambodia and Vietnam.
Habitat: Rivers, lakes, ponds, swamps, rice paddies, drainage ditches, and any other lowland habitat with water, including brackish water. Can be found in the water or on the banks. During the day it hides in burrows and crab holes.
Place in the ecosystem: Jack’s Water Snake eats primarily fish, though may also take crustaceans and frogs. Juveniles of the species are eaten by larger snakes, large fish, monitors, and wading birds.
Danger to humans: Can bite, but rarely does so and is not dangerous to humans.
Conservation status and threats: Due to its broad distribution and ability to live in human-altered habitats, this snake is not considered at risk in Thailand. It is becoming popular in the pet trade, but that has only had an effect on populations at the local level. In nearby countries similar species are declining due to massive collection for food, skins and crocodile feed.
Interesting facts: In September 2010, I came upon a juvenile Jack’s Water Snake in Lumpani Park. Upon viewing and photographing the snake, an American tourist nearby got anxious and stated, “I ask only that you do not harm the snake.” Further inquiry found that he had bought the snake at a Bangkok market and “humanely” released it into the Lumpani Park lakes. He admitted that he had no idea whether the snake was a native species or whether it was venomous.
Releasing a captive animal into the wild, even a native one, is not good for the local ecosystems. Non-native species, such as the red-eared sliders in the ponds, the Norway rats in the streets, and the English house sparrows in the trees, may prey on, outcompete, or spread disease among local species. And even native species that have spent time in captivity are highly at risk for transmitting disease into the wild populations. If you purchase a captive animal and can no longer care for it, please find a responsible person to take over care for the animal rather than releasing it into the wild.
References:
Thai National Parks: Cox’s Mud Snake
The IUCN Red List: Homalopsis buccata
A Checklist and Key to the Homalopsid Snakes
A Photographic Guide to Snakes and Other Reptiles of Peninsular Malaysia, Singapore and Thailand
A Field Guide to the Reptiles of South-East Asia
A Field Guide to the Snakes of South Vietnam
Reptiles and Amphibians of Thailand 
This needs more information about the puff faced water snake
I’m doing my best!
Great post. I had two of these near our farm pond in the outskirts of Rayong. One nearly 140cm+ and the other closer to 60cm and I could not find a species match until I came across this post. Both near the rocky stream that cuts through the property next to ours.