Sure you wouldn't rather see Predator X, who's actually super big? And Ordovician would only be safe creature-wise. The atmosphere may be toxic and such.
^ Yeah, like I said above. Cymbospondylus is truly enormous - Sea Monsters didn't overestimate like with Liopleurodon (in fact it may even be a juvenile?).
Truthfully I’d choose none of these since every one of these seas are full of deadly predators that would clamp their jaws down on humans just to learn what they taste like. If I was forced to choose though I’d say Ordovician as I’d suffocate from the toxic air well before I’d get eaten by the Nautiloids and Sea Scorpions
To be fair, the Predator X would still be more dangerous probably, because its mostly killer jaws and eats plesiosaurs, whereas the Cymbo was probably a fish and placodont muncher.