GR well
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We live locally, so we have been here a few times. At the end of January we went and the waters were full of manatees, spotted eagle rays, black tip sharks, tarpon. Rays and sharks were jumping, manatees were swimming with their babies, it was great. We did the walking trails to the observation tower and took the tram to the sea turtle hospital.
We went again a month later (end of February) when friends were visiting, and there were a decent number of manatee, a ray, lots of tarpon, but nothing quite as exciting.
Fast forward to end of March, when we had more visitors, and there was one manatee and a few tarpon. I was anticipating this, so it was fine.
So, the place is free. And it's nice. The gift shop is nice, the butterfly garden is nice. All the boardwalks and piers are well maintained. The tram from overflow parking/to the turtle hospital operate regularly. The walking paths are clean, the education center is nicely done. The snack bar prices are very inexpensive, and there's been a Kona ice truck every time we have been.
That first time we went, in January, when we thought it was packed, there was another visitor there that we overheard saying that it was nothing compared to a few weeks prior.
So, bottom line - it's a nice free attraction no matter what. But if you really want the manatees, you need to go in the real winter. The power plant keeps the water warm. Other times of the year there's plenty of warm water for them to choose from, so they don't hang around.