Yeah, there's no way they'd go out of there way to make add aquatic animals or aviaries and just give it away... like it or not, expansion stuff takes time and effort and money to produce, just like the base game they're made to be a part of, and the developers are going to expect payment for their work...
Presumably any sort of aquatic expansion would include the ability to construct an aquarium for viewing above ground level or some kind of submarine ride to go under the water to view them... or worse case scenario, they go with your idea, in which case adding aquatic creatures would be practically no work at all; make a lake, toss in an animal, set up a feeding attraction, pure profit!
I'm amazed people still ask this... considering obtaining dinosaur DNA from fossilized mosquitoes is itself scientific nonsense... what's the point of even asking entire premise of this franchise is totally bogus?
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...in any case, that one kid in Jurassic World mentions that they can extract DNA from fossil bones--that presumably includes Mosasaurus bones--which resolves that issue quite effectively... even if it makes even less sense than the mosquitoes...