FG:

If the marble and the bowling ball are made of the exact same stuff then they will hit at the same time. So did the feather and the wrench on the moon.

The thing about "full" sink lines is that they are not the same density. A small line has a much higher percentage of core than a larger thicker line.

You are probably correct about the type II and III's. I don't think it would matter much if you were looking for the center of the water column anyway if one was an inch different.