Multipurpose Room Subnautica
The floating island is south west well 1 notch more to the south then the west if you have a compass and it is floating on the surface as for the precursor portal to the island if you have the energy signature thing that marks the precursor aa or ao anti orbital i guess emplacement that shot down the ship then you all you have to do is follow the path into the mountain.
Multipurpose room subnautica.
Source : pinterest.com