I think they are! But humans are to dense and refuse to believe in anything that they cant prove directly. I think the thought of another strong race scares our kind so we immediately decline the thought of mermaids.
I doubt it. Just like I doubt the existence of Bigfoot or the Loch Ness Monster. A creature can't possibly be that elusive for people to know about it for so long and not finding any bones/fossils/solid evidence. However,most of Marine Life is undiscovered,so there's always a possibility. :P