I would not span 4' between floor joists (drawing didnt show spacing between joists but 16x16 with the amount of joists shown would be 4')with 2x6 flooring.That span is too far apart and the flooring is going to bow up on some floor boards and down on others...treated lumber and the u.v. rays are your biggest enemy in regards to splitting checking and bowing. 24" on center would be max spacing with 16" even better...Spanning 2x6 floor joists 16' long will not be nearly enough..it will have deflection like a trampoline(depending on how close the floats are).2x8x16 floor joists are not big enough either..I would double 2x12 rim boards fastened together,triple 2x12 center beam fastened together in center to make your floor joist span 8'... then you can span 2x6x8 just fine for floorjoists. Fastners should be given proper consideration because the dock will only be as good as your life of fastners. I suggest at least Hot dipped galvanized bolts,nails and deck screws..not electro-galvanizing.The "Maze company" have great double hot dipped deck screws and you can easily find hot dipped galv. bolts(not zinc plated). Since the new generation pressure treated lumbers after c.c.a. treated was removed from residential applications, the chemicals now used are very corrosive to fastners depending on treatment method. Micro-treatments are relatively new and are less corrosive than first generation after c.c.a.Look at pressure treated manufactures recommended fastners.
