I don't really see Spelljammer as SF. There's none of the trappings you would expect to see in even the softest sci-fi; there's no rogue AIs, there's no meditations on the nature of man or transhumanism, there's no cybernetics, there's not even things that flinging yourself around a gravity well. Heck, the cosmology of the setting is crystal spheres! Spelljammer is simply naval fantasy transposed into space; it is full magic-and-dragons-and-big ships but it all happens to be happening in space rather than in the water.