I saw some of those on the 15th of February, when I went to the Grumari beach (I did not go to there to swim or anything like that, I just walked there with my dad for some minutes). They are huge.
@Enzo
Yes, they are really large! These ones in Lagoa Santa are extremely friendly. I imagine they would even let people pet them, but unfortunately it's not possible due to the star ticks that parasite them. Did you see them at the beach? Nice to know they go near the sea.