Well, then I guess my next question would be -- what would she need to be there for? Mammals have been giving birth to their offspring for years upon years... what would Gillian really be needed for?
The ocean's a dangerous enough environment even for species that have a well-established niche there. Reintroducing an extinct species into an environment it's been absent from for over two centuries is going to be an extremely risky undertaking, especially with only a single breeding pair to work with. There's no telling how the ocean has changed in the interim -- the currents, the temperature, the population of various species, the availability of nutrients, etc. It's going to be an alien environment for them, and their learned and instinctive survival behaviors may not be adequate to sustain them. Not to mention the diseases. The ocean is a breeding ground for new strains of virus and bacteria, and G&G would have no immunity to whatever pathogens might have evolved in two centuries (which is hundreds of thousands of generations for microbes). On top of which, their primary predators, orcas, are still alive in the 24th century according to the
TNG Technical Manual, so in the 23rd century G&G would still be vulnerable to their old familiar enemies as well as the new threats of a changed, alien environment. They'll need constant safeguarding and guidance if they're to survive.
Not to mention technological intervention. As someone mentioned above, a single breeding pair is not even remotely near enough to ensure a viable population down the road, because it simply doesn't have enough genetic diversity. Ensuring the long-term survival of
Megaptera novaeangliae as a species would require considerable genetic manipulation to increase the diversity of George & Gracie's future offspring and descendants, perhaps even some hybridization with other surviving cetacean species.
The last we see of George and Gracie, they're swimming off in to the Pacific. Starfleet doesn't seem all too concerned about tracking them or monitoring them at that point. Would they even be able to find the two whales after that?
Why would it be Starfleet's business? We're talking about something that's happening down on Earth. That's not in Starfleet's jurisdiction. Would you expect the US Navy to be responsible for the care of endangered bison in the Great Plains? No, that's for the Department of the Interior, for civilian scientists and conservationists, etc. No doubt there are equivalent Federation, Earth, and regional government institutions, civilian research establishments, and the like who could be responsible for the management of the recovered humpbacks.
How much time passes between the Bird-of-Prey crashing in the bay and the tribunal at the Federation Council? Is it at all possible that a period of months has passed, as Earth recovers from the Probe "attack," while evidentiary hearings are held reviewing the actions of Kirk and his crew? Is it further possible then that the Gracie gave birth to her calf and that the sudden urgency of having Gillian be the only one to look after them was no longer as urgent, hence her signing up for a science vessel?
It could certainly have been months, but as I explained, the situation is going to remain "urgent" for much, much longer than that, because the ocean is full of hazards and the species will remain critically endangered until a sizeable number of whales has been born and reached maturity.
And that could take a long time. Humpback whales give birth to only one offspring at a time, and usually breed only once every 2-3 years. And it takes 5-7 years for a baby whale to reach sexual maturity. So their population is going to grow very slowly even if they don't lose any members to predators or disease or accidents. It would take decades at least to breed a large enough number of adult humpbacks to make it safe to take them off the endangered list.
So this isn't a "fire and forget" situation. Letting G&G out of their tank into the ocean may have seemed like a happy ending in the movie, but realistically it's just the first step of a difficult, dangerous process that will take a long time to see to fruition -- if it succeeds at all. It's not something Gillian could brush off after a few months, it's a lifetime commitment for her and probably for whoever she trains as her successors.
(By the way, TVH's predictions about humpback whale extinction seem, fortunately, to have been overly pessimistic. When commercial humpback whaling was banned in 1966, their numbers were down to 5000 worldwide, but now they're back to 80,000 and are no longer considered seriously threatened. So this is one case where it's a good thing that ST's predictions were wrong.)