Actually, several of the teens/kids at the school Allie is at are members of trading houses. They are there to be trained in their strengths, which is water or air, in some cases. So yes, ships are a big deal. Steam will come, but it's not in a hurry, because with a good practitioner around, sailing vessels are more reliable. If the wind is close enough to coax, a tall ship can keep moving.

I don't want to turn the book into a history lesson, because history will change at some parts. After all -- if there are people with the skill to pop back and forth between continents (which is what it looks like to non-practitioners) why do the wars happen, etc. etc..

Of course, greed is the usual reason a war starts, followed by hate/misunderstanding and fear -- of what you attack, or something at your back that drives you on. So the War of 1812 is coming, because certain greedy men want that war. But some things change. Same with trade, the industrial revolution -- practitioners are not a dime a dozen, so they cannot replace large areas of commerce. But what they do, they do well.

Any particular book spring to mind that would be useful about the shipping? (In English -- my French is rusty and Chinese non-existent!)