Nightowl,
Thank you for sharing your story and the content of your Dad's paintings.
When I dream of 3's sometimes it means mother, father and child. Maybe the 3 swimming dolphins represent what he wished for the three of you on some level, as you did not mention any siblings. My Dad, long since passed, also had a PD. Words were often painful and I still tend to perceive communication from others in a very black or white way. My Mom was a puppet and sometimes co-conspirator. Lots of triangulation. The idea of a dolphin family, and how dolphins communicate...I do not think they commicate aggression... feels very soothing to me. The painting might also show his acceptance if the current situation.
When I read your description of the beach painting...what came to mind was you and your fiance off swimming. When I dream of swimming it is usually about navigating my feelings. You have been learning to do that in a healthy way with your fiance. That might also be what your Dad wished for you, someone to do that with as he could be of limited help because of your Mom's dysfunction.
The above is what I would read into for myself it if I were you.
Try not to get stuck on the meaning of the paintings as he painted them and they are more about him than you. I would say that distancing yourself from them might have been painful for your Dad, but that he has accepted it.
Many blessings to you,
Christine