To traverse or sail between (two places) frequently and regularly. There's a ferry that plies between the island and the mainland twice a week. We have a line of cargo ships that ply between America and Japan several times a month.
ply between (something and something else)
to travel between things or places regularly or constantly. There are a number of small craft that ply between Santerem and Manaus on a regular basis. Our little ship was unable to ply the entire distance between the two islands.