I think a better translation is "cruising."
Why not "the boat sails smoothly"?
Exactly. The Korean sentence is not in progressive tense.
I think a better translation is sailing.
The English translation is kind of weird for this sentence, no?
The ship is cruising is also accepted.
Nope it's correct. You probably wouldn't say it very often though. I grew up on a lake so I don't think it's weird!
"순항 중" "巡航中" ?
Exactly! 巡航中 is "cruising around", 顺航中 is "smoothly sailing", and I'm not saying that in the progressive tense which would be silly . . .
Where did we get ^smoothly^?
순항, meaning 순조로이 항해하다, means that there is no trouble in the movement of the ship, or in other words, sailing smoothly. The word 순항 is a bit of the side of hard words, but it is used often when on boat.
Naver Dictionary definition for 순항 is "cruising, going smoothly"