Comment by nephihaha

There was one major distinction, which was "is it drinkable?" Someone crossing the North American Great Lakes (before pollution), didn't need to store water. But they would crossing an ocean, or the Caspian Sea. It's not like you can use seawater to irrigate most crops (except seaweed and a handful of others).

"Mere" in English doesn't seem to refer to saltwater at all. Nearly always small lakes and mostly in certain locales of eastern England.