I have found that the idea to refer to ebraic to explain ancient Masonic words and symbol lead always to confused and usually wrong explanations. In this case, anciently, cable tow was used mainly in relationship with the penalty for having betrayed the secrets of the Craft and its lenght was much less than 3 miles (from Masonry Dissected (1730) "... to be buried in the Sands of the Sea, the Length of a Cable-rope from Shore, where the Tide ebbs and flows twice in 24 Hours ..."). This phrase indicate some particular places, that is the tidal plains, or mudflats. In the Tirrenian Sea, for example, it would not be possible.
The lodge in which I was raised uses similar wording, but as other brothers have pointed out, this explanation falls flat when considered literal.
To say that it is to designate a distance from shore for the obligation assumes that all geographies are the same. The tidal range is different everywhere. I grew up in a coastal town and at some places, low tide and high tide might only be a few feet different, but in other areas, it could literally be miles (we have a river that floods at high tide and goes miles inland, but which can be walked across at low tide). Combine that with the distance between "shore" (however you define that) and the center of the tidal range, and you could be talking about a distance of a few feet to a few miles.
In short, in my opinion, this meaning does not make sense.
However, many early uses talk about fulfilling an obligation or helping if it's within the length. If we go back to the idea that it is instead a measure of one's pledge to uphold a Mason's fidelity, then this is basically saying that you should attend lodge/answer a summons/help a brother/etc if it is within reason to your obligation.
A real world example: my mother lodge consolidated with another lodge a few years back. The secretary of the new lodge didn't know me or anything about it and I was sent a summons. The problem was, I was living 2200 miles away. I wasn't about to buy a plane ticket to answer a question that could be handled over the phone. My obligation, however, said that I should. By adding the modifier of "if it be within the length..." essentially gives me an "out" if it's not reasonable.
At least, that's one way to look at it. The fact of the matter is, none of us were around to ask the Brothers writing the ritual what their intent was. At this point, it's all a matter of opinion.