My real difficulty with a living Mosasaur is the present understanding that the creature is an air breather, which would make it as visible as the whale population. It simply has not been that visible. What is more its linage is reptilian as is that of the sea serpent that gets by nicely without breathing air. Thus accepting this creature pretty well tosses out the argument of air breathing and I need to know why this was argued in the first place.
In April 1971 the crew of the Kompira Maru saw a "Bug-Eyed Monster" which resembled a large crocodile, but had fins instead of Legs, which were clearly visible as it leaped and dived under the water.
1972 three women were whitebaiting at the mouth of the Orari River near Temuka watched a huge creature wallowing in the breakers about 30 m away from them.
There is nothing to be said to make me think otherwise but personally, I do believe these creatures are out there as there have been too many sightings that are so closely reminiscent of mosasaurs as to be easily dismissed.