Join the club! You aren't confused, you have simply reached the point of riddle that all Waterman descendants reach at some point. There are other researchers, I'm sure, who believe that they have found the lineage for this family, and that might be true. But from what this humble, amateur genealogist has found the answer is that nobody knows exactly who Robert Waterman's ancestors are.
Robert's father was most likely Thomas Waterman, born 1581 in Norwich, Norfolk, England. His mother (arguably) was most likely Mary Bird born (?) in Bishops Stortford, Hertfordshire, England and died in 1628. However, other women have been named as this Thomas' wife.
Robert Waterman's 3rd son (and 3rd child, of 6) was Thomas Waterman. He married Mariam Tracey. I am a descendant of this line.
I suspect some would offer their theories regarding this question...and I hope so. I'd be interested in hearing the thoughts of others and learn the thinking behind the research.
Ducking to avoid flamers...I wish you luck in your search.