Aquaman's no longer in vogue so I find this preoccupation a bit strange. Although I believe it has something to do with a question I gave the kids on who among the superheroes wore a seahorse pendant.
This was right after he asked me to draw a seahorse, his favorite animal nowadays.
Of course you don't see a necklace on Aquaman above. The pendant on the necklace idea was used in Aquaman the Movie (not animated).