It was the mermaids' fault, really. They started it.
Of course, the way they danced was nothing like anything any of the Sunny's crew had ever seen before. Where they were earth-bound (or maybe ship-bound - or at the very least confined to moving in projectile-like trajectory), the mermaids plunged and rose and spun around each other in intricate patterns like glittering butterflies of the sea.
It was a daylight dance, a dance of clear waters and joy, of shelter found and storms conquered.
The urge to join them was so strong that it took three soaked fruit-users, three very annoyed swimmers, and one trussed up cook before they settled down enough to finally listen to the half exasperated, half amused mergirl's suggestion to just dance up there.
It came as no surprise that Brook bowed with a flourish, volunteering to teach the lovely ladies of his crew steps suited to the airy harmonies of the mersong. And they wouldn't have been surprised by Franky stepping forward to show them all his super moves - except these were not the moves any of them were used to.
It even brought the trio of twirling idiots (Luffy and Usopp, with Chopper hanging on for dear life) to a halt.
Following this high, clear melody, so unlike his usual pounding growl, Franky seemed somehow younger than they had ever seen him, and for all that he was the only one moving across the Sunny's deck, it was clear to everyone watching that he wasn't dancing alone.
Brook, with his keen ear for rhythm, was the first to join, and the impressionable parts of the crew broke out into spontaneous applause, because it was cool. (And if nobody thought to comment on the sight of a ridiculously tall, gangly skeleton dancing with a blue-haired cyberman, Robin's smile went a little wider just then than those around her were used to seeing.)
Luffy, of course, had no sense of the intricacies of the dance, but he more than made up for it in enthusiasm and flexibility.
And one by one, they were all pulled in to the melody, to the rhythm, to the empty places in the pattern that needed to be filled. Below and around them, mermaids danced, while the sun shone overhead, and music was everywhere.