3-4. It's flawed in that the plot doesn't really catch fire until the third act. This is the type of film that largely attempts to sustain itself on evocations (a game of pick-up sticks that reveals the natures of its participants; a constantly exchanged phallic knife, etc.) and quiet tension between its three capably acted leads. And the payoff is so good that it almost compensates for the early plotting problems.
I found this as interesting as a minimalistic film where three people share a boat resulting in a triangle where the profile of each character is well delineated, and a growing rivalry between the older and younger men with a tendency to childish competition. I didn't see much cinematography or themes emerge from this one, but others may find minimalistic perfection in that theatrical back and forth between the 3.
Although sheer content of Polanski's first feature film is noting in particularly intriguing apart from a casual boat trip (or not so casual hitchhiking) it offers such a constant tension between protagonists, even the simplest of affairs are experienced as dramatic climaxes on the verge of unavoidable tragedy. It ends with a pun as a tragedy does and does not happen', depending on different characters point of view.
It's a good film, especially towards the end and the ending itself. I was born and grew up in Poland, understand the realities of the early 60's when this film was made and appreciate what Polanski achieved here. Honestly, it's slightly overrated in my opinion though. It's a story about three people on a boat, interactions and tension between them... Definitely a classic now, and worth giving it a try.