The difference between across and through is like the difference between on and in. Across is used for a movement on a surface. Through is used for a movement in a three-dimensional space, with things on all sides. Compare:
We walked across the ice ---- We drove across the desert. I walked through the wood. ---- We drove through several towns.
People swim, and ships move, across rivers, lakes etc.
Fact
There are 44,523,312,694,361,020,971,556,671,544,734,879,370,359,807,003,367,569,358,848,000,000,000,000 ways to order a deck of cards. .. More >>