Predator

The main cast of Predator. Left to right: Ventura, Black, Schwarzenegger, Duke, Weathers, Landham, and Chaves
Major Alan "Dutch" Schaefer and his six-man team, consisting of himself, operators Mac Elliot, Billy Sole, and Blaine Cooper, demolitions and explosives expert Jorge "Poncho" RamÃ­rez, and radioman Rick Hawkins, are tasked by the CIA with spearheading the rescue of an official held hostage by insurgents in Val Verde. CIA Agent George Dillon, a former commando and an old friend of Dutch's, is assigned to accompany and supervise the team despite Dutch's reservations. The team is taken to a remote jungle and begins the mission.
They soon discover the wreckage of another chopper and three skinned corpses, whom Dutch identifies as U.S. Army Special Forces he once trained. Pressing on, the team reaches the insurgent camp and kills every single guerilla, including a Soviet intelligence officer searching through top-secret CIA documents. Dillon, confronted by a suspicious Dutch, admits the mission was a setup to retrieve intelligence from captured operatives and that the dead unit disappeared weeks earlier in a failed rescue. Upon capturing a female guerilla named Anna, the group proceeds to extraction, unaware that they are being stalked by a nearly invisible creature who tracks them with thermal imaging.
Hawkins chases a fleeing Anna when they are both suddenly confronted by the creature. The unarmed Anna is spared, but Hawkins is swiftly killed and dragged away. Dutch organizes a manhunt for his body, during which Blaine is killed by the creature's plasma weapon, enraging Mac. An ensuing firefight fails to draw out the creature, so the unit regroups and questions Anna, learning that their stalker is an unknown creature locals call "El diablo cazador de hombres" or "El demonio que hace trofeos de los hombres", meaning "The devil man hunter" and "The demon who makes trophies of men" respectively. The next day, an attempt to trap the creature fails, leaving Poncho badly injured. Mac and Dillon pursue the alien, but it outwits and kills them both.
The survivors try to escape, but the creature catches up, killing Billy and Poncho, and wounding Dutch. Dutch sends Anna to the chopper alone and unarmed, upon realizing the creature does not target unarmed prey because there's "no sport". While being pursued by the alien, Dutch slides down a hill into a river, goes over a waterfall and ends up crawling through a patch of mud, only for the creature to catch up to him; its cloaking device malfunctions in the water, allowing Dutch to finally see his hidden enemy. The alien, though standing a few feet from Dutch, does not see him and moves on. This helps him realize that the mud he is now covered in is acting as camouflage by cooling his skin and blocking his body's heat signature from the alien's thermal sensor. Now seeking to avenge his men, Dutch uses his knowledge of jungle warfare to craft a series of traps. Covered in mud and armed with improvised weapons, he lures the creature in with a war cry.
Utilizing his preparations, Dutch beats the alien at its own game, disabling its cloaking device and inflicting minor injuries. However, the creature rallies itself and finally corners him. Acknowledging Dutch as a worthy foe, the alien discards its mask and plasma weapon, and challenges him to a hand-to-hand fight, where it has a clear advantage. After being brutally beaten, Dutch narrowly defeats the creature by using a counterweight to crush it. Standing over the crippled alien, he asks, "What the hell are you?", but the creature simply repeats back "What the hell are you?" in garbled English before activating a self-destruct device on its wrist, laughing sinisterly as the count down begins. Dutch flees and takes cover just before the self-destruct device explodes in a mushroom cloud.
Dutch, the last man standing, is picked up shortly afterwards by his commander, General Phillips, and finds Anna in the helicopter.

Question: Why does the creature's cloaking device malfunction?
Answer: Because of the water.