The key to building a sturdy, usable dock ladder is to anticipate the level of the water at the boat dock. If you have tide changes or other types of water level changes you will need to anticipate these changes and build your ladder bottom at a level that will accommodate them. Otherwise, you could find it somewhat difficult and undignified for you to lower yourself from the dock to your boat. Here are a few good pointers.
Step 1 - Measure Boat Dock Height from the Water
Determine the lowest level you can expect your water to be. From the top surface of your dock, add three feet, and mark this measurement on the 1" X 4" boards you'll use to make the sides of your ladder. When in doubt about the length you should make these side pieces, you'll be better off to make them too long, rather than too short.
Step 2 - Measure, Mark, and Cut the Ladder Rails
The top ladder step, or rail, should be far enough down the ladder from the ladder top that people using the ladder will be able to get a safe and secure grip on the side rails as they raise or lower themselves on the ladder. To provide for this safe distance, measure down 36" from the side board tops, on each side board, and mark a spot on each side board where each step will be fastened. All ladder steps below this point should be 12" apart. Count the number of spots where you'll have steps attached. With your circular saw, cut enough 2" X 4" wood pieces, each 24" long, so you'll have enough rails to reach to the bottom of your ladder, once these pieces are mounted.
Step 3 - Attach Your Ladder Steps
Lay out the 1" X 4" side rails on your dock, with the top ends even and parallel with each other. Lay one of the 2 X 4 steps between the 1 X 4 side rails at the top side rail step marks. This 2 X 4 step should be lying on its edge with each end butted up against the side rails. With your drill and drill bit, drill two guide holes through the side rail and into the two ends of the step. These holes should be spaced far enough apart that each of the two holes is made at least ¾" in from its end.
Make two guide holes in each end of the step piece. Then, drive four galvanized screws through the guide holes in the two side rails, two screws at each end of the step. Attach the remainder of the steps the same way, until you have all the steps attached.
Step 4 - Attach the Ladder to the Dock
Onto the bottom surface of the top step attach a length of angle iron, using your drill and galvanized screws. Attach a similar length of angle iron on the ladder step just above the water surface. Finally, attach the far ends of these lengths of angle iron onto the dock side.