I think you'll really struggle with 8mm. I did with only 6mm!
Are you thinking of using st/st screws as permanent fixings? I wouldn't, especially below the waterline. I know there are different grades of stainless but it does tend to corrode away when deprived of oxygen; like when buried in wet wood.
-------------------- The cure for most of life's ills is salt water.
Sweat, tears and the sea.