Ceramic-and-mortar seals on the garum jars were corroded by seawater or removed by the occasional curious octopus, the archaeologists report, but traces of the fish sauce remain inside.
So it's probably just about as edible as it was in the time of the Caesars. In case you're interested, here's a recipe from Gargilius Martialis's De medicina et de virtute herbarum:
Use fatty fish, for example, sardines, and a well-sealed (pitched) container with a 26-35 quart capacity. Add dried, aromatic herbs possessing a strong flavor, such as dill, coriander, fennel, celery, mint, oregano, and others, making a layer on the bottom of the container; then put down a layer of fish (if small, leave them whole, if large, use pieces) and over this, add a layer of salt two fingers high. Repeat these layers until the container is filled. Let it rest for seven days in the sun. Then mix the sauce daily for 20 days. After that, it becomes a liquid.
(Reprinted from A Taste of Ancient Rome)
For this and other ancient Roman recipes (some of which look perfectly nontoxic), see Nova Online.