As it turned out, I could not determine the source of the alarm. However, the oil-pressure gauges looked fine. The engines were coming up to temperature and then staying at the correct temperature. After checking the oil levels in the engine and the outdrive fluid, all was good. So were the coolant reserves. Finally, we inspected the raw-water strainers.
With some hesitancy, I suggested things should be fine but to keep a close watch on the gauges. They had dual Mercury 454s with Bravo drives. They then got in touch with their local service rep, who confirmed that things should be fine, if a bit annoying with the alarm going off every few minutes.
I asked them to ask their service company if their outdrives could “free-wheel,” which allows the boat to run on one engine if the other fails. This is a good thing to know about your gears: Are they OK to free-wheel? If so, you know you can always make it home without risking a lot of other damage to your gears.
They made the 60-mile run just fine, and in the end, it was a defective AIC air idle control valve, the fix for which was found using a diagnostic tool.
The bottom line is to pay attention to the alarm and check everything carefully before proceeding. It helps to have all your manuals on board, to help you learn the different alarms. In this case, their boat had a steady alarm (or an alarm that went off every few minutes), which indicated it was for a less-serious issue.