Neptima's pearl, besides being a defensive weapon, also showed glimpses of future events.
Neptina's origin was never explored, nor how she and Marine Boy became friends.
Marine Boy's mermaid girlfriend, Neptima, was topless (although her hair always covered her breasts). No other children's animated show has ever dared to do this.
Marine Boy's dolphin was actually named Whitey in the Japanese version. It was changed to Splasher for the American market to avoid accusations of racism.
Marine Boy's mother was never shown.