How dangerous was "being cast into the water" in Hammurabi's Code? Did Mesopotamians swim?

Executing a person by drowning would be pretty straightforward, regardless of their ability to swim, just hold their head in water until they stop struggling. But that's not really the answer you're after.

First: The law code of Hammurabi is not really a law code in the sense it's often presented as. It's a monument (quite literally an impressive seven foot tall basalt obelisk with a carving of Hammurabi receiving laws from the god Shamash at the top). It was constructed late in Hammurabi's reign and is a testament to his own interest and efforts in creating a lawful, just (by the standards of the time) society. As he puts in his own epilogue: "...In my shelter I have let them repose in peace; in my deep wisdom have I enclosed them. That the strong might not injure the weak, in order to protect the widows and orphans..."

I hate to quote a movie that came out when kids in your class might not have been alive, but they were more...guidelines, really. There are court records from the old Babylonian period (and many court records from the Ur III period as well, with the preceding law code of Ur-Nammu as a potential reference) and they, with very rare exception, don't make any reference to precedent, or structured laws, or anything really. The judges in cases had pretty free reign to decide guilt and penalties as they saw fit. They were often more lenient than you'd expect from the law code itself, with the harshest penalties (including death) being pretty rare, even in situations that might technically have called for it from the law code. So keep that in mind.

So what's all this about rivers and drowning and ordeals, which I think is really what you're referring to, given your reference to trial by combat?

In ancient Mesopotamia, and in the ancient world in general, and frankly in much of the world until recently, people took this stuff very seriously. They believed in their gods and their ability to influence events in the world, and people's lives. This is something I think fiction and popular history really underplays. Kings, Hammurabi included, consulted the gods on even seemingly trivial matters. Not 'is it a good idea to attack this land?" but rather "is it a good idea to deploy this number of troops on Tuesday the 17th led by this particular general and executing the discussed plan?"

So with that in mind, swearing a oath to a god was no small thing. The gods didn't like people lying in their name and a person doing so could expect divine retribution. Many cases were decided by simply seeing who was willing to swear an oath. Often someone, or their witness in a case, would be unwilling to, settling the matter. It's interesting to imagine someone pushing a lie possibly getting cold feet when it came time to lie to a divine authority, or a witness realizing the risk wasn't worth it. So the gods could be counted on to punish people trying to evade justice, but they operated on their own timetable in their own ways. Sometimes you need an answer a bit more reliably and immediately. That's the idea of the river ordeal - the gods could make their opinion known without much room for interpretation and quickly. If people were already uncomfortable just swearing a false oath, asking them to really back it up under threat of immediate death was even harder. In some cases it seems just escalating to the river ordeal sorted things out. An accused agreeing to it was a strong statement and the accuser may have backed down (and you'll notice false accusations are treated harshly, best to not have your falsehood put on display when the accused doesn't drown - the stakes were high here). It seems, like the death penalty and other harsh penalties, it was rarely actually carried out. The seriousness of it and the shared belief in its effectiveness was often enough to settle issues.

But nevertheless it was actually carried out sometimes. The actual method varied and may have been up to a judge just like the actual application of law was. It's easy to imagine that often the accused was given a simple goal - go in the river and don't drown. Guilty or not they'd probably be just fine, as we know now. Sometimes they were given a difficult task, such as carrying a heavy item a certain distance or staying afloat some distance (in which case failure didn't necessarily mean drowning). One well known example is a letter to king Zimri-Lin (a contemporary of Hammurabi ruling Mari). It's a report of the outcome of a river ordeal at the city of Hit (a popular place for actually performing the ordeal). Taken from Marc van de Mieroop's biography of Hammurabi, it appears to be a property dispute between two places or groups of people. Four people (out of a planned seven) go in the river, first a woman who "came up immediately." Then an old man who swam a distance of 80 "measures." Then a second woman who also came up immediately. Finally, a third woman goes in and drowns. Given the poor outcomes (the man "only" swam 80 measures and one person drowned) they ask to stop, saying they would relinquish their claims to the disputed land for all time. There were three people left to go but at this point they recognized that whatever they may have thought the gods clearly weren't going to side with them. The letter closes with saying the survivors would be sent to Mari to be questioned.

This is really interesting! But as you can see while it provides lots of info it raises lots of questions. This wasn't a criminal trial, this seems like it may have been a civil dispute. It's easy to imagine how two groups might come to disagree on property ownership - contracts can be lost or damaged. Such was the importance of real estate contracts that families sometimes kept contracts that were hundreds of years old in their homes. Obviously there would be no living witnesses to the contract for something that old so the only way, in their minds, to settle this was to ask the gods. That part is understandable, but how did it come to being settled this way? There were other methods of divination available that didn't involve risking lives. Maybe there was animosity between the groups. Since the group ended it after a death after several nonfatal "failures" they may have not expected anyone to die, even if they were in the wrong. This whole event may have been unusual - it was reported to the king of Mari and the survivors were sent to be questioned. Were the people of the one group going in because they were the main complainants? Would the other group have to go next? What was their actual task? The man who swam 80 measures (which I'm not sure how far that is) hadn't swam far enough, and of course one woman drowned trying to do whatever it was (the women who came up immediately may have been poor swimmers but they survived). Who chose the participants from the group? Maybe it was random given they don't seem to have been the best swimmers. You can see at least today this isn't a small piece of river (look up Hit, Iraq), was the goal to swim across a wide part (if so, how did the one man save himself then after presumably tiring? Was the goal to swim it completely underwater?).

The exact details of the Old Babylonian (and Ur III prior to it) river ordeals are murky (pun intended). People willing to swear oaths and risk their lives doing so were often taken at their word so it may not have actually come up often. When it did, the precise details may have varied. And, as seen in the letter to Zimri-Lim, the process could be used as a fast, dramatic way to get an immediate answer from the gods on a matter.