Preliminary accounts say DUKW called, but no response from the tug:
Tour boat calls before crash went unanswered - CNN.com
One of the tug crew took the 5th rather than talk with NTSB.
I know little about the "Ducks" except having seen them around, but from the photo, I doubt that any small passenger vessel would fare well if caught under the rake-bow of that empty barge.
Anyone know how the tug was made up to the barge? If astern or especially if "hipped-up" on opposite side of the barge, then could have been a considerable blind spot.