Re: Skeg Guard?
I would be highly surprised if that is an oem setup to have one way screws shown in your post.
If you remove them as its likely they will be damaged. You may then want to replace them with the correct ones If you can identify the brand of skeggard you shoul be able to order spares.
As far as actually removing them -- that's based on tools at hand, The actual bolt features and your experience. Very hard to say over the Internet.
Can you apply heat to them?
Can you get vise grips on them?
Can you drill a bolt head off?
Other methods?