Complex sentence: The second semi-navalised variant of the Seafire, and the first to be built as such, was the Seafire F. Mk IIc which was based on the Spitfire Vc (which in itself had several major refinements over the Spitfire Vb).
Split into simpler sentences: The second semi-navalised variant of the Seafire, and the first to be built as such, was the Seafire F. Mk. IIC based on the Spitfire Vc (which in itself had several major refinements over the Spitfire Vb).