His birth reg on freebmd shows his mothers maiden name alongside the entry - Merrifield
You can then usually look for a marriage between Waterman and Merrifield on freebmd but none found in this case
www.freebmd.org.ukHis mother may have been previously married and if so - she would have married Waterman under her married surname.
Any birth certs will show mothers birth surname - not her married one
There is also the possibility they were not married but the birth was registered in his fathers name