I agree with regisRquoi on some points, but disagree on others.
I think the difficulty of the game is pretty good to begin with, games need to be difficult. Maybe tone down just a bit from the start, but the general difficulty is good.
I think the oil spill is fine. I noticed it looked like oil from the first time I saw it, slipped in it and figured it out right away, I think most players will.
As far as the building goes, I wouldn't mind seeing a progression bar. Either a small one right above the player, or a larger one somewhere on the GUI. Possibly directly below the health and junk indicator or right above the gadget buttons.
If you had a progression bar for building, then I think the cancel build could be as simple as moving. If the player is going to get attacked, their natural instinct is going to be to move.