I feel like you really can't have a proper maritime film festival without some version of Treasure Island. I also feel like some animation would be good; perhaps Ponyo? Though I'm blanking on what English-language animation might suit.

I am reminded that I've been meaning to ask you if you've ever seen Ondine or any episodes of LXD, in particular the shipboard episode or the mermaid episode. They all have great dancing, but I think you'd very much love the mythos of those two.