The transparency in the tail over the girl isn't directly noticeable, and especially not from a distance - it needs to read first and foremost as what it is rather than what it could be considered to be, when it's so subtle it doesn't come across too well. I myself had barely noticed the transparency in the tail actually until you'd told me. But you could also accentuate it just by having something different behind the shark. Maybe more mermaids, some rocks or corals or even some other creatures! These are just suggestions though, so take them how you will xD