I, for one, would not want to say that. What I would say is that Loch Ness was not created until long after all the giant marine reptiles had been long extinct! And if they were not extinct, how exactly did it find its way thorough miles of solid rock to reach this isolated body of water!!!
In this article about the “Great Glen”, the fault line in which Loch Ness was formed:
“Erosion along the fault zone during Quaternary glaciation formed the famous Loch Ness. The fault is mostly inactive today, but occasional moderate tremors have been recorded over the past 150 years.”
The article for the Quaternary Glaciation:
… we read: "The Quaternary glaciation is the last of five known glaciations during Earth’s history. The other four are the Huronian glaciation, Cryogenian, Andean-Saharan glaciation, and Karoo Ice Age. "
Lakes: “The Quaternary glaciation created more lakes than all other geologic processes combined. The reason is that a continental glacier completely disrupts the preglacial drainage system. The surface over which the glacier moved was scoured and eroded by the ice, leaving a myriad of closed, undrained depressions in the bedrock. These depressions filled with water and became lakes.”
“Very large lakes were created along the glacial margins. The ice on both North America and Europe was about 3,000 m (9,800 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Ice weight caused crustal subsidence which was greatest beneath the thickest accumulation of ice. As the ice melted, rebound of the crust lagged behind, producing a regional slope toward the ice. This slope formed basins that have lasted for thousands of years…”
And from an article on Loch Ness itself, we read this interesting item:
“Loch Ness … is a large, deep, freshwater loch in the Scottish Highlands extending for approximately 37 kilometres (23 miles) southwest of Inverness. Its surface is 16 metres (52 feet) above sea level.”
The ocean would have to elevate more than 52 feet to give access to this landlocked body of water.
“It is one of a series of interconnected, murky bodies of water in Scotland; its water visibility is exceptionally low due to a high peat content in the surrounding soil.”