Jack Shaftoe saw a "high tide mark" on the beach at Algiers (page 4 of the Confusion). Months later, "the tide was quite low", when he visited Malta (page 209). When I was reading Julius Caesar's De Bello Gallico in the original Latin in 1944, I learned that there were no significant tides in the Mediterranean Sea. Caesar's unfamiliarity with the phenomenon of tides caused him problems with military operations, on the Atlantic coasts of Gaul and of Britain. [Undoubtedly, that lack of knowledge also contributed to the failure of his attack on Qwghlm (page 256 of Cryptonomicon).] I cannot recall now whether the absence of Mediterranean tides was mentioned by Caesar himself, or only in the classroom discussion. For sure, there is no Latin equivalent to the English word 'tide'. (See an online English-Latin dictionary, such as http://www.freedict.com/onldict/lat.html .)
As a matter of fact, there are noticeable vertical tides at the head of the Adriatic Sea (near Venice), and at the 'corner' of the Gulf of Gabes. Both places were nearly uninhabited at the time of Caesar. Vertical tides have been measured at Malta, but they are inches at most. The scuba diving establishments there advertise: "Throw away your tide tables." (See http://www.aquatours.com/gozo/malta_bugibba-diving.htm .) There are also significant horizontal tidal currents near Malta, and especially in the straits of Messina and of Sicily.
Unfortunately, Neal Stephenson built this 'invisible' tide into the story. It is only implicit at Algiers, in that the galley, on which Jack was a slave, had been beached to have its barnacles scraped. That is very easy to do on a tidal beach, but without tides, the slaves would have to pull it out of the water by hand. When finished, they would either drag it back into the water, or dig a channel to bring the water to the galley. None of this was mentioned by Stephenson. At Malta, he explicitly needed the piers to be higher than the galleys, in order to advance the story.
Surely, if the basin of the Mediterranean were different (in length, width, or depth, or in some combination), there could be significant tides almost anywhere in it. My knowledge of proper oceanography is so scanty, that I can't even start working on this problem.