The, for lack of a better term, boat movie, has always been a great way to explore psychological tension inside and between characters. Das Boot qualifies I suppose, but ideally the sea craft is of small size and occupied by only a few people, often two men and one women. And when I say psychological, I also mean sexual, which is what really propels the prime examples of this unofficial genre. You would definitely have to name Rene Clement's Purple Noon, Hitchcock's Lifeboat, as well as Dead Calm and the first third or so of L'Avventura.
Yet the best, and sneakiest, of them all is Roman Polanksi's debut, Knife in the Water. Polanski gives us characters, clues, snippets of dialogue, and arguments, but he doesn't give us much information. For those who enjoy piecing together a character's inner world, this film is a dream come true. It's also an incredibly bold choice for Polanski, not just from story-wise, but geographically. With the exception of an opening scene, in which a man, Andrzej, and his girlfriend, Krystyna, pick up a hitchhiker (who uses a rather daring technique to get a ride), the whole movie is set on the very small boat (I think only in Hitchcock's film will you find one much smaller). But Polanski never makes it dull; when he's not dealing with the characters, he focuses on the technical aspects of the boat itself. The characters seem to be constantly at war with it to make it move, which if anything gives the film some real-world grit. In one scene, Andrzej and the hitchhiker have to run through a jungle of reeds to haul the boat through a narrow, shallow trench, and in another they get stuck in a shallow patch of water. But if this sounds like some sort of boat adventure movie, it's not. These people are just out for a day in the sun.
The movie is obviously a love triangle in that the husband seems to love his wife and the hitchhiker is at the very least physically drawn to her as well. The boat is such a small and intimate place, and since the characters are often in just swimwear, we especially notice their bodies and the shapes their limbs take as they sit or stand in various positions. One visual motif Polanski falls in love with is the formation of a triangle through the limbs of the body. Try it with your arm or leg and you'll see it's pretty easy. In one great scene, we see Andrzej talking with the hitchhiker while Krystyna is swimming. He leans close to the hitchhiker and forms one of those triangles with his arm, in the middle of which is Krystyna splashing about in the background.
Yet there's more going on here than a three-way romance. I haven't mentioned why the hitchhiker ends up on the boat with the couple, partly because it's one of those instances where Polanski seems to relish letting the viewer pry apart the motivations behind the action. Andrzej initially tells the hitchhiker that he can tag along once he learns he's never sailed a boat before. Yet we're immediately on the alert because this seems like it's supposed to be a romantic day on a boat between Krystyna and Andrzej, yet the latter clearly is putting the interests of the hitchhiker before his girl's. And yet it plays out smoothly because we see from the beginning that Krystyna has eyes for the young hitchhiker. If this couple is having problems with their relationship, they're definitely not out to mend them on this particular day.
From Andrzej's perspective, there seems to be two possible incentives for inviting this stranger aboard. As I said, he may be having some romantic problems, so perhaps by inviting another man to join them he can in some way prove something to Krystyna. Based off his intimidating and domineering attitude towards the hitchhiker that escalates throughout the film, he would be forcefully affirming to her a strong masculine identity that perhaps had been waning of late. But what seems to be just as legitimate a cause is that Polanski is responding to the post-modern ennui that permeated Antonioni's L'Avventura. In that film, young people seemed trapped in perfect bodies, wandering through a malaise where beauty transcended soul. They needed a kick like the one Andrzej provides by inviting a complete stranger onto this tiny boat. Something just needed to happen.
Yet through all these psychological avenues one can wander down with this film, it's still, without any question, a great thriller. The "knife" in the title, which I won't deny has some sort thematic implication, refers on a literal level to a large hunting blade that the hitchhiker carries. It's the most frightening thing in the film because it seems so out of place. The hitchhiker talks about how he uses it when hiking on land, where he could cut branches or maybe even kill a bear. Clearly it's something that doesn't belong on the boat, and Polanski seems to be suggesting by its forceful presence that the only use it might serve is on somebody. And by revealing little about the characters and what they're made of, there's really no way to tell what they're capable of doing. Now that's a great way to make a thriller.