A fin on the back sounds kinda cool but watch out for shark fishermen.
Actually, I thought about that- you could cause a lot of consternation if you painted it black or grey, and swam off a beach! Not actually a good idea. In my case, a cheesy wooden prototype attached to a weight belt that is just wood coloured wouldn't trick anybody.
I think if you wanted to paint it, something like bright orange or neon green or yellow would avoid misunderstandings, and probably be a safety feature.
P.S.- some kids are probably already having fun with shark fin hats like these ones:
Swim Gear TYR SHARK FIN JR SILICONE SWIM CAP - DJ Sports