Dor or Dwr derives from pre-roman Britain meaning water. This is when the Durotriges were pre-dominant in the area I assume. The Roman name for todays town of Dorchester was Durnovaria and there is reference to Dorotrioum (the romans had to build an aquaduct to supply the hill town). Whether these two names were used in conjunction to differentiate between settlements I am not certain but Durnovaria Vindocladia is Badbury to the north of Wimborne Minster. The Set is a shortening of a word used by the saxons which has come to us as settlement. It is most likely that the seaside dwellers and the seaside lands were joinly denominated as dwr or dor rather than one being the cause of the other.