So you have your incredibly talented cast, your suited-to-the-film director, now you need to know where to film your epic mermaid movie of awesome. We'll go by your type of movie and how you can play off your location to the best advantage of a film.
For your typical epic high fantasy sort of film (it can be one with a darker portrayal of merpeople or it can work with a lighter tone as well), you'll want to go to where most epic fantasies are filmed: New Zealand. You've got plenty of beautiful locales to do your scenes on land (if you have any, which odds are you will) and many more for the water bits. If you don't believe me, just watch the Extended versions of The Lord of the Rings a couple times.
For a dark take on an Urban fantasy, I'd go with Boston or Washington D.C. Washington is a little harder with the water scenes, but there are great bodies of water in the proximity. Boston might be a bit easier in that aspect however.
Or, if you're going in the vein of the beloved "Splash" New York of course :) Another interesting though would be Canada. I've always thought of setting some mermaids up in Canada. Maybe that's just me.
Either way, lemme know what you think guys!
Much love and sonic screwdrivers,