A spaceship crashes onto a water planet, an octopus type creature hooks onto the passengers and creates a false reality for them. Most of the passengers actually died. It was from so long ago, and that's all I remember 😭
A vague description, but it sounds like one of the plot points in The Ellimist Chronicles in the Animorphs series by K. A. Applegate.
A spaceship crashes onto a water planet
A group of survivors from a planet called Ketra (which is destroyed by invading aliens) crashland on an aquatic moon. This includes the protagonist, who is known both as Toomin and his gaming nickname Ellimist.
an octopus type creature hooks onto the passengers and creates a false reality for them. Most of the passengers actually died.
A gigantic, many-tentacled sea creature known as "Father" absorbs the minds of the Ketrans, killing everyone but Toomin. Father keeps Toomin alive while telepathically communicating with him in a sort of false reality, using the minds of his dead friends to talk to him.
It was from so long ago, and that's all I remember 😭
The Ellimist Chronicles was released in 2000. A lot of other stuff also happens in the novel, obviously; I wrote another answer covering the book here, see if any of the rest of the plot sounds familiar. Here's the cover:
This sounds a lot like Michael Crichton's "Sphere". Scientists uncover an alien vessel that crashed into the ocean ages ago and investigate it, building an underwater facility. After entering the ship, some of the crew's thoughts begin to manifest in reality. One crew member who had just read 20,000 leagues under the sea, was convinced that a squid would attack them, and soon enough, an intelligent squid soon does. It becomes unclear what is real and most of the crew dies, this is as far as I remember.
I apologize for poor answer formatting, I'm used to a desktop but stuck with mobile temporarily