Tugboats aren't the only option--- you could use on-shore rope wrangling and pulling, you could use sails, you could use focused orbital mirrors to melt ice on one side as a propulsive force... and so on.
This idea of pushing, pulling icebergs was investigated by ppl much smarter than me (Glaciology/Maths dept. Uni Melb) to see if it was at least feasible. The idea has some merit, the physics (moments of inertia) get in the way. Make for an interesting experiment to prove how.
