What does 6 short blast of a barge horn mean when boating?

Usually the tug towing the barge will sound it's horn or whistle. Most barges are unmanned.

5 or more short blasts of the ships whistle is the danger signal. If you hear this while boating you are likely in the way or crossing the path of the tug and tow, and there is risk of collision. Tugs with tows are much less maneuverable than a small boat and usually must stay in the dredged navigation channel.