Charlotte Barras set up full-back Waterman for a well worked first after England turned over the visitors near their own try line with full-back Katy Mclean missing the conversion but adding a penalty soon after.
But McLean soon had the unfamiliar sensation of taking a restart after Natalie Marchino crossed for the USA - the first team to score against England in the World Cup and only the second this year after France in the RBS 6 Nations.
Yet once McLean set up Merchant for England's second try with six minutes before the break it became one-way traffic with Waterman making it 20-5 at halftime.
The outstanding Scarratt got the bonus point after the break and despite scrum-half Kim Magrini replying for the visitors, Penrith and the combative Alphonsi added a convincing gloss to the scoreline.
Elsewhere, Ireland wrapped up their second consecutive bonus point victory but ultimately it was not enough to take them through to the semi-finals.
Their convincing 37-3 defeat of Kazakhstan meant they finished second in Pool B but it was Australia who progressed to face England in the semi-finals courtesy of their superior goal difference.
Philip Doyle's side made the best possible start in their bid to meet the necessary points difference with full-back Niamh Briggs going over in the second minute.
It took a while for the second try to arrive, courtesy of No.8 Joy Neville, and from then on it was one-way traffic.
Scrum half Louise Beamish, Orla Brennan, Eliza Downey, Kate O'Loughlin and Neville all went over but ultimately it was in vain.
Scotland also recorded a morale-boosting victory in their 32-5 defeat of Sweden with centre Lucy Millard and No.8 Donna Kennedy both grabbing two tries apiece.
Wing Katy Green got the other try as Scotland join Ireland in the fifth to eight play-off places.
Wales though ended their tournament on a downer with a heavy 41-8 defeat to reigning champions New Zealand despite a much improved performance.
Winger Caryl James grabbed Wales' only try but star of the show was