If electronic communications don't work or you don't have them at hand, the Coast Guard lists a variety of ways to signal distress to vehicles close enough to see you, such as: ...

Once the boaters were located from the distress call, Coast Guard aircraft directed the 720-foot, U.S.-flagged Coquí and a nearby fishing vessel, the Bonanza, accompanied with smaller fishing vessels, ...

we remain cautiously optimistic that they are not related to any persons or any vessel in any distress or any kind of trouble." ...

But what distress signal would be universally understood? Before the 1906 agreement, various countries and organizations each decided their own signals for shipping. One of the most widely adopted was ...