Think of it like Tetris. push the boats into the right shipping lanes. Once a line is filled horizontally, it'll 'unload', just like completing a line in Tetris.
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What throws me off is when I turn the tug left or right. My instinct is to press Up to go forward. Instead I need to keep pressing Left to push left. Then if I'm in the wrong place, the boat starts to canter. Then I panic and I can't fix it and it's frustrating for me. It's not long before the river is jammed.
I love the UI, and the character development with the different wakes is creative and beautiful.