I do believe they exist but are in deep hiding. There are so many cultures that have them depicted in their arts. At the same time the myth about sea creatures rocking boats and killing sailors and other people on them has been found that that creature is also real. That is the Giant Squid. If you watch River Monsters, Jeremy Wade goes after beings that are supposedly myths and finds them. Just because we don't see them does not mean that they do not exists. There is too much art about them for them not too.
Jeremy has also found an elusive creature in the deep that may be the Loch Ness Monster--not the fake one they have on the internet, but the one the locals have seen. That being is the elusive Greenland Shark and he did catch one in the fjords of Norway. We haven't even been to 1/10th of under the oceans and in the rivers to even make a calculated guess about what creatures are in our waters.