Roy Scheider.
He left the show when the writers went sideways. Scheider enthusiastically joined the cast when at the beginning, the writers produced scripts that dispelled common myths about oceanography and taught some science. It started out well.
In later seasons, hoping to boost ratings, the show writers went all kinds of Discovery Channel, trying to introduce Mer-people, UFO's and aliens and other obnoxious plot twists. Scheider felt that the show had moved away from its roots and not into a particularly good direction and quit.
The role was re-cast to Micheal Ironside - another great actor, but doom for the show. It got the axe not long after.
Of course... any review of recent shows that no one learned their lesson from any of it...






