thats what they say about the lochness monster!
Nessy, Chessy (Chesapeake Bay Monster), the "monster" in one of the Great lakes are all believe to be pleasaurs. Which, if one thinks about it, could very well still exist.
Supposedly the asteroid that hit the Earth created a huge dust field that made it too cold for dinosausrs to exist. If they were cold-blooded. But snakes, turtles, crocodilians all survived. So why not smaller boided saurapods and therapods? Especially ones that exist in the water. Evidence is that if the reptilians of today survived, then there could be other species that have survived.
All that is needed is determination, deep pockets and technology. One could use satellites to track large bodies organisms in these waters--and especially in fresh water areas, where large bodied animals are rare. Zoom in on those, go out with boats and capture it.
One could capture big foot by using infrared or using body heat sensors. If all humans are out of the zones being looked at, then 7-9 tall upright body heat image would stand out. Tranq the thing capture it. There you have Big Foot.