This brief guide will let you know how to get Fungal Samples in Subnautica.
Starting from the coral tube sticking up out of the water, you will need to head about two bumps to the left of the northwest until you reach the northwestern Mushroom forest.
It is recommended to have a sea moth, a high capacity O2 Tank, a sea glide, and you wil also need a knife.
The only threat you might encounter in the mushroom forest is the bone shark; you can either avoid or kill using the knife and stasis rifle.
The coordinates for this location are (-919, -177, 508).