If you want it to last a long time and be rock solid then I would suggest the following.
1. Replace the wooden beams with fiberglass beams. Strongwell (Structural Shapes and Plate - Strongwell
) is a good source for composite beams and panels.
2. Either buy composite panels or fabricate your won using something like Coosa board (Specialized Structural Panels - Coosa Composites
). Ideally, make the deck as one piece or as a second choice 2 pieces and bond the panels to the composite beams. That will create a solid, rot and leak proof deck. Coosa board is available in sizes up to 5'x12'. It would make a great base for Strongwell's non-skid panels.