As Shakespeare said :"There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy. "
Up till about 1930 nobody believed that a species of fish could have survived for millions of years without anybody having found any sign of them. And then some fishermen caught a coelacanth off the coast of South Africa. And then again, they were believed to be extinct, untill recently another couple were caught.
If you're asking about sea monsters then I believe there are definitely a whole lot of creatures in the ocean that we don't know about and they could qualify as "monsters". If you're asking about the Loch Ness monster in particular, there is no real evidence that can conclusively prove that it does or does not exist (although as Oron pointed out the chances of it existing undetected are very small).