@ Forge :
You didn't sound insulting, but I take that like a subtle way to say that I should spend my mapping time on more important projects, anyway that isn't a bad advice at all

Wherever, about sunstroke what bothers me is some cramped areas inside the ship and the size of the map, it is huge. I have a cool plan for it, but haven't had the motivation to make it.
About gameplay, after you reach the ship cockpit you'll find a tripbomb, it is supposed you need to use it to descend the cliff, next you'll find a yellow keycard in the green truck at the right, the slot is in a door at the very left of the map, the switch in that room unlocks the big EDF door.
In order to access the underwater spaceship you need to find a green "crystal" in the red caves at the river bottom at the left of the map.