Shabbos, Chapter 11, Mishnah 5. As we mentioned in the previous mishnah, the sea is considered a karmelis, and on that basis our mishnah comes to discuss throwing and carrying between the sea, the dry land, and ships.
One who throws from one domain to another in the sea and on dry land:
"Min hayam layabashah" - one who throws an object from the sea, which is a karmelis, onto dry land, even dry land that is a public domain.
"U'min hayabashah layam" - from the dry land, which is a public domain, into the sea.
"U'min hayam lasefinah" - from the sea onto a ship, which is a private domain.
"U'min hasefinah layam" - from the ship, which is a private domain, into the sea.
"U'min hasefinah lachaverta" - from one ship to another.
In all of these cases the ruling is "patur" - exempt, since there is no transfer here from a private domain to a public domain.
Ships tied to one another:
The mishnah continues: "Sefinos kesuros zu bazu - metaltelin mizu lazu" - if ships are tied to one another, one may carry from one to the other, and even at the outset. The Gemara explains that the case is one of two ships belonging to two different owners, and for that reason the question of eiruvei chatzeiros arises: a person may not carry from the domain owned by one to the domain owned by another unless they made an eiruv between them. Perforce, then, our mishnah is dealing with a case where the two owners made an eiruvei chatzeiros, and therefore they are permitted to carry from one ship to the other.
Ships that are not tied:
But the mishnah adds: "V'im einan kesuros - ein metaltelin mizu lazu" - if they are not tied, one may not carry from one to the other, even if their openings face each other and they are pressed right up against one another. In this case even an eiruv does not help: since there is nothing binding them together, they are liable to drift and separate from one another, and that separation nullifies the eiruv and voids its force. It turns out that one who carries from one ship to the other is transferring from a private domain to a private domain by way of a karmelis, which is forbidden rabbinically, and the eiruv is of no help to him for that.