중 = Middle
You can think of this construction being translated literally as:
In the middle of doing something
Another form of progressive tense
Verb + 는 중이다, be in the middle of V-ing
배가 순항 중입니다 -
[Lit. The boat is in the middle of cruising] -
The boat is cruising along...
And where is 'smoothly' then..? 0_o
배가 순항 중입니다 - The boat is cruising along.
Since "To cruise" is by definition, to move or proceed speedily, smoothly, or effortlessly [Merriam-Webster dict.]
=> 배가 순항 중입니다 = The boat is moving smoothly
The boat is cruising.
So what is 순항 means ?
순 = smooth
항 = sail, navigation
순항 (where 순 is used as a kind of postposition), noun = smooth sailing = cruising