In all my life, in all my travels, in all of my experiences, I have never seen a mermaid not wearing a Seashell Bra. It's like a uniform for those people (fishpeople?). Whatever the reasoning or the cause is, it's obligatory: If you're a mermaid, you are expected to wear a Seashell Bra. Otherwise, how are people supposed to tell what you are? (Oh yeah, the fish part of your body) Well, still. Point stands. Mermaids should dress like mermaids.
And even if you're not a mermaid, you're just an island girl or a hula dancer of some kind, there's still a pretty good chance that if you're near water in any professional capacity, you should be wearing seashells like they're clothing. It's just common sense. The Seashell Bra is the perfect accoutrement for your mermaid costume this Halloween.