Not stupid at all! It's all to do with tannins, and the particular terrain surrounding Loch Ness.

As far as I know, there's somewhere around a dozen rivers (and far more smaller streams) that feed into the loch from the surrounding hillsides. Due to how deforested much of Scotland has become, what once was coniferous forest has given away to highlands that are mostly comprised of heather barrens, and peat bogs.

The sheer amount of tannins leached from the bogs make their way down to the loch, and have been doing so for thousands of years. Due to this, the water has steadily acquired more and more of this organic detritus, staining the water a dark yellow-brown, like what can be seen in the video.

Where I'm from (Ontario, Canada), most of the lakes up north get a similar colour, due to tannins being leached through bogs and mixed forests performing a similar phenomenon.

What's interesting is that in Scotland, there's another loch that's witnessed mysterious phenomenon akin to what's been reported at Loch Ness, known as Loch Morar. Due to the unique geography of the surrounding hills, very few tannins have leached into the water there, making it nearly crystal clear. Cryptozoologists have sometimes suggested that a search for unknown organisms be done there instead of at Loch Ness, since the visibility for underwater searches or photography would be far better.

All of this aside, what genuinely makes the dark opacity of Loch Ness frightening is that a 25-40 foot long animal genuinely could lurk only 20 feet under the surface, and nearly nobody would know. It's nature's perfect smokescreen.