Sometimes he whirled down the incline of a wave as if he were on a hand sled.
The man is clearly in a context of desperation or at least difficulty, stuck out at sea on a life preserver. The imagery brings one of a childlike delight or a fun situation. However that is contrary to the actual situation. The man clings to a life preserver, trying to survive, not have fun, but nature rolls on, indifferent to this man's plight.
