I am planning to make a large expedition with my boat. Does travelling by boat reduce my food bar? If so, how much (compared to walking/swimming)? And does this actually depend on the difficulty? I am playing on 'easy'.
Boating does not inspire hunger. You can use it to cross thousands of blocks (meters) without any need for food. On the other hand, you can only cross 133 blocks per haunch of meat by swimming (0.015 hunger/block) and 200 blocks walking (0.01).
Difficulty only affects hunger as to what happens when you're starving. Your health drops on easy until it hits 5 hearts, normal drops to 1/2 heart, and on hard you die.