I got given this board and one screw in the turtle box is rusted and just wont come out. 
I have broken about 4 screw drivers and used heaps of wd - 40.
I was thinking about drilling the head of the screw off. When i take out the other screw, the fin is wobbly so the fin isn't jammed.
Drill the head off it and pull it out from below.
I suggest drilling a narrow pilot hole first to ensure the bigger hole you drill remains centered.
Looks like from that 1st pic that there is still some groove for the screwdriver to engage in? And then the second pic is unclear.
If there is enough screw head groove remaining that you can at least apply some torque before it slips:
Use the right sized driver and quality :) put a wide elastic band over the screwdriver tip, then push down HARD and turn whilst somebody taps the screwdriver handle down with a hammer - like about half as hard as you'd hit to drive a nail into pine. The shock loading alone can often be enough. If the head is stuffed bad enough nothing will work and drilling will be ur only option, just thought I'd mention the above as a last ditch effort.
Drill a small hole and use an "easyout". You can get one from local hardware....left hand tapered thread. should do the trick if no luck with hammer and screwdriver.
find who used a mild steel screw instead of stainless and .... { insert punishment }
We used to use hammer and chisel, bang the side of the head until it turned, less technically refined, but good for anger management.
being in a hole about 75mm deep makes grinding / cutting / easyouts a bit hard.....
Rubber bands help fill in the gap a bit and increase friction. Does not always work, and neither does the shock from hammering. Put all together with a quality driver and it is a last ditch effort that may work.
I find impact drivers work pretty well, they are essentially designed for just this situation.
