Regarding Bigfoot, the Patterson-Gimlin film is a really clear shot of either a walking ape or some guy in an ape costume. But that isn't some "squint and maybe it looks like something" shot. It's clear as day. The debate is entirely if the film was staged and faked but the footage is really good.
The interesting thing about the Patterson-Gimlin film was it was filmed back in the late 1960s which was exactly the same period of time that the Planet of the Apes films were made. If you've ever watched the planet of the apes movies, you can see how horrible the costumes are, but this was state of the art for hollywood special effects at the time. So if the Patterson film was a hoax, it is a very impressive one because it is far superior to what even hollywood could pull off in those days.
The Loch Ness Monster however is pretty much been proven as a complete hoax. The whole thing started based on a photo that was later proven to be faked.
I agree. There is 99.9% certainty that the Loch Ness monster doesn't exist, because the whole entirety of the Loch has been methodically swept over with Sonar trying to find it. If there was anything as large as Nessie supposedly is, those sonar sweeps would have found it for sure.
Another issue is that Loch Ness is almost completely devoid of fish, so what would an animal that large eat? There isn't much food there. The waters are also very cold, which a reptile such as a plesiosaur wouldn't be able to survive in.
However, some have argued that Nessie is an apparition or something like that instead of a living animal. I guess if that's the case it would explain why the sonar wouldn't have detected it. Its definitely not a living plesiosaur, but it might be the lingering ghost of one that had lived millions of years ago and just never "passed over" for whatever reason.
Aside from that, its probably just a combination of hoaxes and known animals like seals and fish and so on.