Inertia.
The berg or water or whatever form it is in doesn’t want to move, so the back end of that sack or kiddie pool or whatever is going to have to be really strong.
Remember that thing where 90% of the iceberg is below the water? Really deep shipping channels for the biggest ocean-going ships are 45 feet deep. So you’d have to wait for your giant berg to get down to where it sticks less than 5 feet above the water before it will fit. And the smaller it is, the faster it melts (because it has more surface area proportional to its volume).
I think some problems could be … reduced by sculpting the berg quite a bit before travel, and if folks can figure out a reasonable way to turn the berg into useful water at the end of its journey, there would probably be purpose-built berg-tugs with the engines from aircraft carriers.
But I think the pipeline idea might be the most practical: harvest the bergs close to their point of origin, shipping water is a much more easily tackled problem. That tech was solid when Rome was built.
Interestingly, transporting ice from frostier regions use to be a very, very big business. Frederic Tudor made a fortune shipping ice in ships from Boston to the American South, Europe, and even to India. Ice was packed tightly together and insulated with wood shavings. It was stored in ice houses at the various locals.
Tutor also imported fruit cooled with ice from the Southern climates into Europe and the American North.
Okay, Boston isn’t the Arctic, and blocks of ice aren’t an iceberg, but it’s still pretty cool.
My wife volunteers at Fosterfields, a 19th-century gentleman’s farm in New Jersey. One of the many businesses the farm was in was cutting ice off the pond.
I’m talking about the weight of the material, not the weight of the water it’s holding. Dragging around several tons of teflon is no easy feat.
Powers &8^]
People who don’t work in shipping tend to think of aircraft carriers as the gold standard for big ship engines. Actually aircraft carriers are showponies compared to large tankers or bulk carriers. Yes, the engines of an aircraft carrier (which are often turbines) are powerful but they are aimed at light weight and speed. If you just want to output a lot of power economically over a long period of time, a medium or low speed diesel out of a large tanker or bulker is a much better bet.