Wow, that's a really unique theme! I've never run into one like that before. I think it looks fun, and not any different from having a pirate wedding or a Renaissance wedding or anything else (well, in terms of content, obviously, but not in terms of what's acceptable). I'm not a big fan of weddings where people dress or act "in character," like characters from movies or tv shows, because I think that a wedding should be able you and your husband declaring your commitment (not Uhura and Yoda, or what-not), but I have no problem with costumes--after all, white poofy dresses and tuxes are costumes of a sort as well. Almost every wedding has a "theme," whether it's conscious or not (coordinating colors and styles are a theme, if a less noticeable one than a Hawaiian luau).
I do think it's nice to indicate on the wedding invitation if there's any sort of theme that makes the wedding noticeably different from the "norm," though, just so that people are prepared ahead of time (even if they aren't expected to wear costumes or do anything differently). That way somebody who has a deep and abiding fear of catfish in any form could make their excuses for this wedding.