You don't have to go to the Dunes, mobile vehicle bay fragments can be found in boxes in the Kelp Forest, and that and Seamoth fragments can be found in the Grassy Plateaus. By the Floating Island I mean the one with the actual bases and not the one that is underwater with bonesharks, and it's above the Grand Reef (I suggest watching a video on how to get there). I wouldn't go to the Dunes at least yet because it's incredibly dangerous and to deep considering what you have currently.