I thought they were supposed to have fixed this…? I have a massive plessie army at the bottom of my ocean.
Total insanity.
Anybody know how to make it stop? 
You can make adjustments by adding in a custom spawn for deep water and add them to it, but if you do that I am pretty sure you have to make adjustments to the water height when they spawn in. You can also make a custom darkwater spawn and remove them from the spawns. If you used Remap NPCs to replace the vanilla game plesiosaurs, then revert that and use vanilla. Pretty much that is all the options I know of.
You could edit the darkwater spawner to spawn fewer plesios per run. As a huge dino, I think it is only intended to spawn 1 at a time.
Nah, neither suggestion will work. That spawner is just plain broken.
Correction I have used the methods I recommended with my own mod with success. So, I know my suggestion will work, just not ideal honestly as it would be better if we could just limit their spawn rates, but dark water is messed up and when you try and limit it nothing happens. If you make a copy of dark water and remove plesiosaurs from the spawner they will not spawn at all.
If you allow vanilla plesiosaurs to spawn and not modded variants, you should have no issues with plesiosaurs. At least with my mod that is the case, but the moment I try to use my modded variant of the plesiosaur regardless of what I actually change and use the dark water spawner for it, they spawn out of control.
Thanks for the info guys.