Man i am pretty sure there more monster in the deep sea than meet the eye. There an episode where they found a Lake under the ocean. WTF... a lake under the ocean, how is that possible. http://youtube.com/watch?v=CQBgJVmeAeg&mode=related&search= check it out. Call the deep 1 alway the way to part 5. Part 5 is where they discover a lake in an ocean. Plus part 4 has some prehistoric shark that haven't change. Cool stuff.
According to Fantasy/Horror author Steve Alten the Bloop is from a giant Anguilla species. Pretty decent book titled THE LOCH. Lots of fantastical fun.
Sea monsters do exist. That is if you consider giant squid, oarfish, basking sharks, Nomura jellyfish and whales "monsters". I don't believe the megalodon or any of those prehistoris marine reptiles are alive anymore.
I think it is perfectly plausible that there are large "monstrous" yet undiscovered species of fish and inverts in the ocean. We have only explored one percent of the deep ocean bottom so far. Almost every deep dive records new species.
Many people will argue that any undiscovered species of large air-breathing animals (mammals and reptiles) could not exist in the ocean, as they would have been spotted by now. I used to think this was a reasonable theory, but consider the various beaked whale species (Ziphiidae). There are about two dozen species, ranging from 4 to 12 meters, and weighing from 1 to 15 tons. These are not small animals, yet any information about their lifestyle and population numbers remains almost unknown. Some species have never been seen alive.
Also consider that despite our best efforts, we do not know where great white sharks and blue whales go to breed. The largest animal ever to exist on the face of the earth gets lost in the ocean.
I would think that if any of the large sea reptiles were still alive we have have skeletal evidence that dosen't date back 65 million + years. I have no doubt that there are tons of fish and inverts out there yet to be discovered but I don't know if I would consider any of them monsters. You mention the beaked whales. True, we don't know much about them but we do know they exist. A marine reptile the size of a whale would have been found by now even if it were only dead. The ocean is large but as we expand our presence here it is getting smaller. We have miles of longline and other fishing material drifting the oceans and air and water craft constantly combing the surface. If it lives near the surface or has to breath air we would know about it.
I don't consider any animal a "monster" myself. I also don't really believe we will find lost mosasaurs or anything either, but I am always willing to admit there is a possibility we may find something unexpected.
Not all fish that die float above water. Some whale die and sink to the bottom of the ocean floor. Watch my link and they will show you a dead whale getting eating by a deep sea bottom shark. Most animal in the while have longer life range than we human do. So something might be moderate size don't mean there isn't one that super size. Monster or not, we haven't even scratch the bottom of the ocean so it hard to tell there isn't any monster or ancient species down there.