Currently, disabling sea monsters and aggressive animals in the creative options does not affect the creatures that have already spawned. I created a world that had a sharks in close proximity to my spawn right after the world was generated, and whenever I use this save, I end up with several sharks circling my builds while I'm testing them, with sea monsters and aggressive animals having been disabled for over 15 minutes since they spawned. While this is technically a feature request, I would argue that the current behavior is not the expected behavior, so this can also be seen as a bug.