Manta's Gift by Timothy Zahn

When Matt Raimey had his accident, he thought his life was over. He never dreamed, in his wildest fantasies, that he'd end up in a spot like this. In the toxic atmosphere of Jupiter, born into the body of an enormous creature that looked like a cross between a manta ray and a dolphin, he is living a new life, unlike any humankind had previously experienced.

An unbelievable turn of events, it gave him a reason to live, to survive, no matter what happened . . . but every second chance comes with conditions and responsibilities. And as those who brought him to this strange destiny have their authority stripped from them and he discovers the truth that only he can know about the giant alien creatures he now calls family, this man reborn as the one they now call Manta suddenly isn't sure he wasn't better off before. . . .