Different Types of Movement
Although normally characters will be moving upright on two legs, the same mechanic applies to all forms of movement. This includes swimming, climbing and flying. Characters are assumed to be familiar with any form of movement they are physically able to do, so all humans can swim and climb but not fly.
The only difference between other forms of movement and standard movement is the resistance of each type of terrain and the consequences for fumbles. The movement resistance for swimming through calm water is three per 5' square. Swimming through fast-moving water or rough seas is much higher. A steep hillside has a movement resistance of three whereas vertical cliffs have a movement resistance of at least five per 5' climbed. Climbing a ladder has a movement resistance of two but climbing a rope has a movement resistance of three. Flying creatures always face a movement resistance of one per square unless there is severe weather or turbulence.
Because the resistance for climbing and swimming is high, the chances of a movement fumble increase. A fumble whilst climbing may result in a fall; one whilst swimming may start the character drowning. If the movement resistance is three or less the character can still choose to mosey, avoiding the risk of a fumble but limiting the amount they move.
Characters who perform a non-standard but physically achievable type of movement, such as a human crawling, double the movement resistance of each square.