i was having this debate with a friend. with how undiscovered it is, what could be the craziest but most plausible?

Chelodina or aka snake-necked turtles. This species is different from the softshell turtle both in size and a permanent long neck sticking out. The softshell are much larger and don't have their neck out unless threaten. While the snake-necked turtles have their long neck out always. And they only grow top 30 cm. If these were bigger and can live all over the world both fresh and saltwater, the whole plesiosaurus sightings would be debunked for good.