So... uhh what do the choices entail?

[hollowleviathan]

Several of the choices have an immediate effect on the game, even if they don't create 'branching' endings.

Spoiler below!

You have the choice, technically, to set the sunken boat to explode before you plug Catherine into the escape boat. This dramatically changes your dialog and understanding of the situation.

Also, choosing to not poison the WAU heart causes Ross to become upset at you and pull you back into the room to attempt to possess your body. I didn't retry the game differently but I assume this plays out differently if you decide to kill it.

Deciding

Spoiler below!

if the WAU lives or dies,

I think, is one of the biggest choices in the game, even if the ending cinematic doesn't address it.