Interesting thought on adding more creatures… I’d thought of it, but didn’t suggest it because the simplicity of a single creature representing the overall threat seemed effective. Perhaps one other creature in the background would help convey the sense of a group, rather than a single creature. Overall, I like what you’ve done with the scene though, as you mentioned, the boat on the water still isn’t really reading that well, even with the two other creatures heading toward it.
As to the creature designs, I'd say you still might want to consider getting rid of the suckers on the tail. They're still a little bit distracting (though not nearly as much as before) on the one in the foreground, and I'm not sure what their function would be in relation to sinking ships. Of course, if you are really attached to them, keep them in, but removing them might help the composition, even if you change the location as mentioned above.
As to making the creature more threatening in appearance, the reason it may be reading as "bland" or "boring" right now is the fact that every line of the creature is smooth and rounded. In general, these shapes tend to give things a softer, gentler feel. For contrast, consider the [angler fish](http://www.dribl.com.au/pics/Nemo%20Fish.JPG) from Finding Nemo. There are a lot of jagged, uneven shapes (teeth, etc) that add to its menacing impression. For your creature, perhaps the tail could have a more ragged shape (minus the suckers), the top fin could be thinner and spikier, emulating a fish and adding a shark-like feel... basically, break up the countoured, rounded form with irregular, sharp ones. Play around with it a bit, and see what you come up with.
Hope this helps, and keep at it! 
P.S. - You mentioned that you’d removed all additional lighting, but there’s still a bit of green phosphorescent light on the tail… not sure if this was intentional or not. If the light is from the background elements, it would be more white/blue in color.