Seas and rivers are neither private domain nor public, but rather the in-between “karmelis,” so if we throw an object from sea to land or vice versa on Shabbos, we are exempt from bringing a sin offering for violating a Torah prohibition, but we have transgressed a decree of the Sages. If we’re in a boat on a river, and we wish to draw in water on Shabbos, the manner depends on the height of the boat’s side over the river. If we’re using a 4×4 handsbreadth constructed private domain with a hole in it for this purpose, we don’t also use the hole to toss out waste water, because we don’t want to pollute our drinking water. This is like the Chofetz Chaim’s rationale for prohibiting gossip: don’t pollute the same mouth we use for praying by using it to speak lashon hara. One may carry between boats that are tied together on Shabbos, and the rules of creating the tie or repairing it are similar to the rules for creating or repairing an eiruv on Shabbos.