Shabbat 100: 4 Mishnayot and a Hanging Question

Tannaitic halakhah... What happens when you throw an object 4 amot within the public domain? 4 different scenarios that fulfill this description.... When would you be obligated to bring a korban? When exempt? Plus: Now add bodies of water into the equations. Why is the water itself considered a karmelit, as compared to the public or private domain? Also, inconvenient passage vs. inconvenient usage. Plus: ships and boats, when you can carry from one to the next. How do we count the depth of the area around the boat? Where are the 10 tefachim?