Lay out 2-by-4 boards on their edge in a rectangle the exact length you want your ramp. Measure down the length of the longer part of the rectangle and place in another 2-by-4 every 16 inches. Paint each board with several coats of paint. Nail together the boards with the 16-penny nails. Cut a sheet of plywood and lay it on top of the grid you have assembled and nail it to the top with the 6-penny nails. It will resemble a bookcase at this point. Screw in the eye bolts into each corner, taking care not to crack the wood.
Turn over the ramp bottom. Fill each joist with Styrofoam and secure it with waterproof caulk, with the exception of one open area on the end, either the top or bottom of your "bookcase". Cut two 2-by-4's as supports for the top of your ramp. Nail the 2-by-4's into the inside between the joists and then fill the remaining space with Styrofoam and caulk.
Saw off the top of the 2-by-4 supports at the angle you wish your ramp to be. Save the scraps and nail them to the bottom of the ramp. Lay a sheet of plywood on the support and nail it to each support. Nail plywood to each side of the ramp, cutting to fit on each side. Paint the plywood panels with several coats once you have attached all sides.
Run rope through the eye bolts and place the ramp in the water. Secure the ramp to a dock or another sturdy anchor point. Examine the ramp to ensure that the bottom is fully submerged while the ramp itself is mostly out of the water. If necessary, secure old tires to the sides of the ramp to add buoyancy, threading them through the rope.