By these Hanjia, it would be to sail, not the sail, right?!
It doesn't mean the sail that you hoist, which is apparently 돛. It means the sail that you go for. I think "(the) sailing" would have been clear.
No, 해 in this context is rooted in Hanja meaning "the sea," so "to sail" would be "항해하다"
"순항" and "항해" mean almost the same thing.