step 1. be on a boat.
step 2. make sure the boat is unlocked. (see figure 1a.)
step 3. place the mouse cursor 1-2 inches away from your character in the direction of the plank (must be facing land. see figure 1b. below.)
step 4. push the right mouse button (see figure 2a.)
step 5. if done properly then you should experience the results below (shown in figure 2b.)