Manus Provincial Police Commander Chief Inspector David Yapu issued the warning following the arrest of youths who stole from MV Manus.
Owned by Papindo Trading, the ship captain and his wife were held up, assaulted and had items worth K4,779 stolen on Sunday.
The vessel was anchored near Rara Island when the suspects, who are known to police, held it up.
One arrest has been made, with more expected to follow.