The Asian Water Monitor occurs throughout much of southern Asia, from India in the west to the Philippines and the Indo-Australian islands in the east. They can grow up to 3 meters, but average out at 1.5 meters.
The Asian Water Monitor is an extraordinarily large lizard, without much noticeable size difference between sexes. They are very dinosaur-like in appearance, usually with dull grey skin with yellow speckling, which is far more noticeable among juveniles. In the wild, they are not unknown to reach at least 7ft in length, however in captivity they usually tend to average out at 1.5 meters.
The Asian Water Monitor is, unsurprisingly, from Asia, with concentration in the Indonesian islands, usually in rivers close to the sea. They spend a lot of their time cruising the water, and the rest of it they spend basking in the hot midday sun. They live in a very hot and humid environment, but they do spend the hotter part of the day in the shade beneath the trees.
Experience is required, but the water monitor is by no way a particularly dangerous or aggressive species. A lot of room is also needed for them to live comfortably.
They need a large pool or pond, roughly twice as wide as they are long and three times as long as they are long. This needs to be three times as deep as they are deep, which is actually quite shallow for such a large lizard. They require not a lot of land space, a riverbed scenario, achievable with the riverbed sand from exo-terra. A few plants along this wouldn't be out of place, making the animal feel more secure - if it can't see you, it assumes you can't see it.
A basking zone with a high UVA/B output wouldn't go amiss; somewhere along the bank (keep the light above the plants though). The general temperature should be fairly warm, average temperatures in the wild would hit around 45-50C (113-122F) tops, but about 35C (95F) should do them nicely. The high humidity could be achieved by placing a steam-producer under the pool, obviously protected by a cage so the lizard doesn't hurt itself. I have seen similar devices for frog's cages, these would probably be the best bet. They prefer the water to be calm, so waterfalls won't be required, although could be added for rainforest effect. The water should be sieved at least twice a week to remove defecation, and the riverbed should be checked over at these times as well.
The vivarium will, doubtless, need to be made from the basic bits from the local DIY shop, at least 2-3ft tall and able to fit over the pool with room to spare on the riverbank, at least enough so the lizard can stretch out comfortably. Juveniles can be kept in a somewhat smaller enclosure; a 6/3ft glass tank should be enough for them, with a large bowl of water sunk into the middle for them to soak in. This will do it until they get to a larger size, when they should really be moved to the larger enclosure.
Monitors are primarily carnivorous lizards, and the Asian Water Monitor is no exception. A few full grown chickens or similar birds per week should suit the adults, with a few full grown rats to vary the diet. They will eat dead meat, they do scavenge in the wild, and will eat it if you leave it for them in a bowl. However, it is far better for inter-species relations if you hand feed them, allowing them to take the food from your hands. This will build up a level of trust between the animal and yourself, meaning that the lizard will not feel scared or stressed when you pass by, and similarly, neither will you. Fish in the pool will not only make for an attractive scenario, they will also break down the algae, eat the mosquito larvae, and make a rather cheap and effective hunting opportunity for the lizard. Juveniles should also be fed locusts, as they are a rather good supplement food. The food could be dusted with vitamins, but this shouldn't be necessary.
Handling is recommended, at least with the juveniles, so they become used to their owners. Adults will probably not object, but holding onto a 1.5 metre lizard is no easy task. They probably won't object to your dipping your feet in their pool, but that's down more to ones own sense of adventure and safety. They are powerful lizards and should be respected, but they are not terribly violent.
Overall, while they are quite high maintenance, they are beautiful and impressive lizards which are very rewarding. Give it time, and you will have a pet that is as loyal as a dog, but up to 50 times as unusual and interesting.
As a side note, if you do keep dogs or cats, use a firmer lid on the cage. A monitor this size would be quite partial to a bit of fresh canine.