UFO's, USO's and origins?

 As the world ended a few months back and all the shops shut I've been getting a bit short of reading material (I read a lot normally) So while reading my way through my old stash of books I found a few UFO ones from way back (1960's?) One of these has some interesting suggestions in it..
 You've all heard of UFO's, some however may not have come across USO's, these are unidentified submerged objects.  Water born UFO's really.   Reports of UFO's flying in and out of water go back forever.
 More recently it's raised it's unidentified head again with both Russian and American subs detecting these objects, in a few cases speeds of over 300 mph have been recorded.  Now we just dont have anything that do those speeds underwater. I think around a claimed 200mph is the best we've managed, that only for a short burst. 
 The problem is water density, it's just hard to push through. The best answer so far is creating a bubble in front and around the torpedo so it's moving into an air pocket (supercavitation) rather than water. This works fine for a short distance but uses a lot of energy to maintain the bubble.
 Back to the book and the connection.  If USO's do exist and may be lined to UFO's, indeed may even be the same objects in or out of water the question is why are they mucking about in lakes and the oceans?
 The book suggests they live there.  It goes on to suggest that the Ufonauts (their name for them not mine) may have always lived in our oceans.  Now this isn't quite as crazy as it sounds.
 We know a lot more about the moon and mars than we know about the deep ocean, only a fraction has been explored. It's not impossible to imagin a race that lives deep underwater, one that for the most part avoids contact with the hooligans on the surface who fight each other all the time.
 Actually on a planet thats surface is largly water covered a race that live in the sea makes more sense that one that lives on the very much smaller area of dry land.