I repeat again APU in 1549 case didn't do anything else. In dual Engine flame out checklist the APU start is way down because in ELEV EMER. you cannot start APU above 25000ft. and because of high speed windmill relight is available at higher altitude. He started it from memory and it would have been life saver had a flamed out Engine restarted. But Engines had not flamed out they remained crippled. Normal Law or alternate law makes no difference to ditching. In normal law due to dropping speed he got into alpha prot and couldn't flare sufficiently and in alternate law he would have got stall warning and he would have pushed the nose down but the result would have been same, hard impact.