IF SHIPS ARE TIED TOGETHER, ONE MAY CARRY FROM ONE TO ANOTHER. IF THEY ARE NOT TIED TOGETHER, THOUGH LYING CLOSE [TO EACH OTHER], ONE MAY NOT CARRY FROM ONE TO ANOTHER. (100b)
In other words, there must be some kind of physical connection holding the ships together.
This becomes clarified with an extra-mishnah text (a beritah):
it was taught: If ships are tied to each other, one may combine them and carry from one to another. If they become separated, they are prohibited. If they are rejoined, whether in ignorance or willfully, accidentally or erroneously, they revert to their original permitted condition.
This becomes much like an eruv ; a string tied around an area of virtually any size – but unbroken – which creates a kind of artificial “private space.” Tying two ships together makes them one “private space” – and even if they become separated and then rejoined they become one again. And, according to Samuel, they can be tied together with anything that will hold – a chain, a rope or even a ribbon.
Keep that rope handy!