Refers to a route taken by ship or the journey itself. Somewhat literary or formal; in everyday speech, 航路 (こうろ) is more common for 'sea route' and 船旅 (ふなたび) for 'voyage'.
I traveled all the way from Japan to America by sea.
This sea route flourished with trade long ago.
Compound of 船 (ふね, 'boat') and 路 (じ, 'road, route'). The reading ふなじ is a regular rendaku form.