Hi Lesley and Jean
Don't think Ann Thompson is correct, I looked for children born 1810 - 1825 named Waterson and Watterson on the Isle of Man to William and Ann, and found some born to William and Ann Millar, Ann Thompson and Ann Skinner, they have the same names, but the ages are all wrong, and no Richard.
I found a Mary born Halsall, Lancashire in 1821 but also married there in 1841, so not your Mary. I was surprised how many Mary Lea's born in Lancashire in that period, so still suspect that is where Halsall comes from.
Richard and Mary on the 1841 census are living in Lionel Street Bannester Court, Liverpool, along with Thomas Owen, John Owen and William Williams. They are all aged 20-24 and as it is 1841 no relationship shown. Mary, Thomas and John are shown born Lancashire, Richard and William outside of census county.