There are two different ways to play River King 2. You can choose to play as the Kid or as the Teen. The Kid's game starts out at the lake and makes his way toward the ocean, and the Teen's game starts out at the ocean and moves toward the lake. Until certain events occur, each can't access the other's maps.
Starting as the Teen is easier then playing the Kid. The Kid starts at level 1 but the Teen starts at level 8. The enemies on The Beach are more difficult then the ones at The Lake, so the extra levels help. The Teen has less events to do before he is allowed access to the lake maps.
Since each of the two games are a little different, two different walkthroughs have been made.