Water monitors are excellent swimmers. They have a long flat tail that works like a paddle, pushing them through the water. They can hold their breath underwater for over half an hour if they need to. On land, they can run surprisingly fast on their short, strong legs, and they climb trees too.
They aren't fussy eaters. A water monitor will happily snack on fish, frogs, birds' eggs, snails, and almost any leftovers it can find. In Bangkok's Lumphini Park, the lizards have become so used to people that they wander between picnic blankets, looking for fallen scraps.
Their long forked tongue flicks in and out, just like a snake's. They aren't sticking it out to be rude - it is how they smell the world around them. Each flick brings tiny scent particles to a special organ on the roof of their mouth that tells them what's nearby.
Even though they look fierce, water monitors are usually shy of people. If you walk past one in a Thai park, it will almost certainly head straight back into the water and disappear under the surface. They are protected animals in Thailand, and people are not supposed to bother them.