Joe, likely that's right and I wasn't there myself but well, maybe...

I'm going by a 50-year-old memory of what Lomax reported his source singer told him. In the booklet that came with the record. Disappeared some decades ago, unfortunately and LofC tells me they can't replace it. I just listened to the two sounding calls and context doesn't help. I absolutely agree that "mark twain" = two fathoms. As you write, this was considered the safe depth. But my memory says that it was the benchmark, the yard, as it were. They'd use a light heaving line (a twine) with a weight over one side of the boat then the other and call out the depth each time. The two-fathom knot was "one." One unit. Each depth had its own call or choice of several calls. "Quarter less twain" was common and "half twain." I think the first meant 1 1/2 fathoms (a quarter of mark twain less than mark twain.) Then it might go "8 feet" or other depths in just feet. Apparently it was just pragmatic and they didn't care beyond a certain safe depth - seemed to go from mark twain to "no bottom" (ie no 2 twain or anything.)

However, I believe Belafonte used the specific line "Marking on the twine is four fathoms."

The line was certainly marked in unite less than 6 feet. At least every two feet, maybe one foot.

Anybody got the book? I like this explanation better, wrong or not.