Sightings of Sasquatch-like beings go back far too many centuries and across too many cultures that never encountered each other for it to be mere mistaken identification. These peoples weren't stupid, they lived off the land and knew the difference between various animals that inhabit these places. There's certainly some truth to the stories. Just like the world flood myth, it's too far-reaching to not be based in truth.
Nessie, on the other hand, I find myself a bit skeptical of. Of course, I'm certainly open-minded toward the possibility. After all, the giant squid was once relegated to the world of myth and legend until one just washed up on a beach one day. But my interpretation of the evidence I've seen so far points more toward no than yes.