But he didn’t have any energy. Sirius would probablg drown since he wouldn’t have the energy to swim. It’s not enough to just get out, you also need to get back.
@Gryffindor pride28 in the movies Sirius was killed (hence the green jet of light), but in the books (which are what I base everything on on this Fandom), he's hit by an unknown curse, but it doesn't kill him, it either rendered him unconscious, or made him unable to move his body.