This is a sci-fi novel, fairly short, and I think fairly old (50s/60s, but might be earlier). The author is well-known, but possibly for other genres as well as sci-fi, and definitely anglophone. I'm fairly sure it's the first book of a trilogy.
This man travels to another planet, and things are poisonous (I'm not quite sure about that). Plants have grown to enormous sizes there.
The bit I remember the best is that he's rescued by another race, who go out on the water in canoes a lot, and look a bit like otters or seals. He lives in their village for quite a while and learns their language. I think a lot of their words start with "br" or similar, and their life is at quite a slow pace. There's a battle against some other creatures at one point and an elder is killed. He refers to them a lot by the name of their race.
There's also a part where he travels over a mountain pass, although I can't remember why, and stops in a cave midway, where he meets a different race, who possibly gives him oxygen (although I might be making that up).
I think it's very well known, but I can't get search terms close enough to find it!