Take a bus to the residential district. After that, take a right. You'll come across a big ledge. Call the sea-creatures to get over it, and then you will come across another big ledge. Do the same thing with the sea-creatures. After all that's done, you should see a path. After going on that path, you will come across two more paths. One that goes off left, and the other that goes off into the right. Take the left path. You should then see one of those big metal doors that you have to hit the ball on it to get past it. Activiate the water ball, and open the door. Then all you have to do it follow the path and you should eventually see Sandy's Dome House.