This article tries to predict scientifically how many sea monsters may be around.It's worth a read.
Extract: Speculation as to the nature of large unknown aquatic animals has generally occurred in the absence of quantitative data and relied almost solely on eyewitness testimonial. This need not be the case. I (Paxton 1998, abstract here) estimated the number of unknown large open water marine animals awaiting discovery by science based on an assumption that the scientific description rate for unknown large aquatic animals from 1830 could be extrapolated into the future. If this is true then the cumulative species description rate can be modelled as a rectangular hyperbola (Figure Two) and an estimate of the number of large unknown open water marine animals could be made.