You've built a floating dock and it's tied off to the shoreline. While you might not mind the short step from land to water, some of your guests might not be as seaworthy as you are and demand a gangplank down, so they may make a more dignified entrance. With a few thick planks, some support, and a bit of rope to keep the ganplank tethered to the shore, or a hinge to keep the gangplank tied to the dock, your guests can weave their merry way aboard, without fear of getting their feet wet.
Cut eight 1x6 planks to the desired length of the gangplank, using a circular saw.
Cut one 4x4 to a length of 46 inches, for each three ft. of length for the gangplank. Cut two additional 4x4s to a length of 72 in. to secure and stabilise the gangplank.
Pre-paint or pretreat with a lumber waterproofing treatment all surfaces of all the planks and lumber before assembling the gangplank. Allow the coating to cure thoroughly before assembly.
Clamp one end of the 1x6 planks edge to edge with carpenter's clamps, keeping the ends of the 1x6 planks even. Slide a 4x4 x 72 in. piece under the 1x6 planks. Use a hammer and nails to attach one end of each of the 1x6 planks to one of the 4x4 x 72 in. pieces, nailing downward through the 1x6 planking. Repeat for the opposite end of the 1x6 planking, with the remaining 4x4 x 72 in. piece.
Clamp the centre of the 1x6 planks together, slide the remaining 4x4s under the 1x6 planking, spaced three ft. apart. Nail the planking down on to the 4x4s.
Add the metal handrails to the sides of the gangplank using the hardware included with the handrails.
Caulk all nail holes and all nail and screw heads on the gangplank with marine silicone caulk, after it is assembled.
If the dock is a floating dock, you'll have to compensate for the vertical movements of the dock in the water. One method is to attach four casters to the end of the gangplank that will end on the dock and then secure the gangway to the same posts that tie the floating dock to the shore with chain or rope. The other method is to hinge the gangway to the dock and allow it to move freely on the shore. You can paint the surface of the gangplank with a coat of non-skid paint to ensure safer footing.