Shabbat 101 - Ships in a sea
If on Shabbat on throws an object from the sea to the dry land or from the dry land to the sea, he is exempt from bringing a sacrifice. As usual, "exempt" means that the Sages prohibited it, to put him farther away from a transgression, but this is not a real violation. But why would this be prohibited? The sea is not a public area where many people walk!? - Nevertheless, it is similar to a public area in the vastness of its expanse. Such areas are called "karmelit," as we discussed before , and carrying there is prohibited.
Likewise, he is prohibited from carrying from a ship to the land, and even from a ship to a ship. However, if two ships are tied together, then carrying between them on Shabbat is permitted, because both represent private areas. If they become untied, then carrying between them is again prohibited, even if they are still together - because it is very likely that in the near future that will separate.
Art: Andries Van Eertvelt - Two Ships at Anchor