I remember reading a short story online set in alien land with ocean and algae/fungus and the people begin dying due to infection but we later learn they get to live on forever in the algae. The atmosphere of the story was very good.
You are possibly remembering the Novella "Turquoise Days" by Alastair Reynolds. It was compiled in the volume "Diamond Dogs and Turquoise Days" published in 2003.
It does indeed feature an ocean world, algae/fungus that absorb the consciousness of the people who come to inhabit the planet. The story also features an infection introduced to the planet to try and kill the algae off. You can read a full synopsis on wikipedia here... not quite what you describe but a close enough match.
The algae is actually a sentient alien life form known as a "Pattern Juggler".