The character of Frankie was originally played by Lance Henriksen before being replaced by Jude Ciccolella for undisclosed reasons. There was an incident during filming where Henriksen was struck in the face with a bottle, but it was not made public if this was the reason for the actor switch.
Captain Avery comments on Murtuagh dodging a bullet by never getting a boat. In every Lethal Weapon movie there is at least 1 scene that revolves around Murtaugh's boating hobbies.