6mm between planks. (Apologize for the lack of information)
They recommend ~40cm spacing between supports. The wood is only manufactured in lengths 120cm x 12cm wide.
That is what we used, 4"x 2" galvanized for our deck. In Mendip's case I would run 2 from shore to the cement pillars. Weld across the ends then weld 2 more long ways to the shore for 4 total and then lay the wood planks. Your deck would be about 120cm ( 4ft ) wide. As for side railing. I would make that all out of galavinized box steel as well. They make a narrow width wood that is also thinner. That's what we used for the side wall for our pool area. You can space them any gap you want. Mendy could even get fancy and take some round pole steel and weld in some fishing pole holders that can double as beer/Ya Dong holders.